Cleanup various Zarr-related build issues

# Description
Remove various obsolete build options. Also do some code movement.

## Specific Changes

* The remotetest server is sometimes unstable, so provide a mechanism
  to force disabling calls to remotetest.unidata.ucar.edu.
  This is enabled by adding a repository variable named
  REMOTETESTDOWN with the value "yes".
* Fix CMakeLists.txt to use the uname command as an alternate
  to using the hostname command (which does not work under cygwin).
* Remove the JNA stuff as obsolete
* Remove the ENABLE_CLIENTSIDE_FILTERS options since it has been
  disabled for a while.
* Fix bad option flag in some github action .yml files: change --disable-xml2 to --disable-libxml2
* Collect globalstate definitions into nc4internal.h
* Remove ENABLE_NCZARR_FILTERS_TESTING option as obsolete and replace
  with ENABLE_NCZARR_FILTERS
* Move some dispatcher independent functions from libsrc4/nc4internal.c to libdispatch/ddispatch.c
* As a long term goal, and because it is now the case that --enable-nczarr
    => USE_NETCDF4, make the external options --enable-netcdf-4 and
    --enable-netcdf4 obsolete in favor of --enable-hdf5
    We will do the following for one more release cycle.
        1. Make --enable-netcdf-4 be an alias for --enable-netcdf4.
        2. Make --enable-netcdf4 an alias for --enable-hdf5.
        3. Internally, convert most uses of USE_NETCDF_4 ad USE_NETCDF4 to USE_HDF5
    After the next release, --enable-netcdf-4 and --enable-netcdf4 will
    be removed.

.github/workflows/main-cmake.yml

@@ -2,6 +2,9 @@ name: NetCDF-C CMake CI - Windows
 
 on: [pull_request, workflow_dispatch]
 
+env:
+  REMOTETESTDOWN: ${{ vars.REMOTETESTDOWN }}
+
 concurrency:  
   group: ${{ github.workflow }}-${{ github.head_ref }}  
   cancel-in-progress: true
@@ -160,4 +163,4 @@ jobs:
           run: |
             cd build
             PATH=~/tmp/bin:$PATH ctest . --rerun-failed --output-on-failure -VV
-          if: ${{ failure() }}    
+          if: ${{ failure() }}    

.github/workflows/run_tests_cdash.yml

@@ -4,8 +4,11 @@
 
 name: Run CDash Ubuntu/Linux netCDF Tests
 
-on: workflow_dispatch
+on: [workflow_dispatch]
 
+env:
+  REMOTETESTDOWN: ${{ vars.REMOTETESTDOWN }}
+
 concurrency:  
   group: ${{ github.workflow}}-${{ github.head_ref }}  
   cancel-in-progress: true

.github/workflows/run_tests_osx.yml

@@ -300,7 +300,7 @@ jobs:
 
       - name: Configure
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests --disable-xml2
+        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests --disable-libxml2
         if: ${{ success() }}
 
       - name: Look at config.log if error

.github/workflows/run_tests_ubuntu.yml

@@ -4,7 +4,10 @@
 
 name: Run Ubuntu/Linux netCDF Tests
 
-on: [ pull_request, workflow_dispatch]
+on: [pull_request,workflow_dispatch]
+
+env:
+  REMOTETESTDOWN: ${{ vars.REMOTETESTDOWN }}
 
 concurrency:  
   group: ${{ github.workflow}}-${{ github.head_ref }}  
@@ -171,7 +174,7 @@ jobs:
 
       - name: Configure
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests  --enable-doxygen  --enable-external-server-tests --disable-xml2
+        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests  --enable-doxygen  --enable-external-server-tests --disable-libxml2
         if: ${{ success() }}
 
       - name: Look at config.log if error
@@ -266,7 +269,7 @@ jobs:
 
       - name: Configure
         shell: bash -l {0}
-        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests  --enable-doxygen  --enable-external-server-tests --disable-xml2 --disable-shared --enable-static
+        run: CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} LD_LIBRARY_PATH=${LD_LIBRARY_PATH} ./configure --enable-hdf5 --enable-dap --disable-dap-remote-tests  --enable-doxygen  --enable-external-server-tests --disable-libxml2 --disable-shared --enable-static
         if: ${{ success() }}
 
       - name: Look at config.log if error

.github/workflows/run_tests_win_cygwin.yml

@@ -10,6 +10,7 @@ env:
   SHELLOPTS: igncr
   CHERE_INVOKING: 1
   CYGWIN_NOWINPATH: 1
+  REMOTETESTDOWN: ${{ vars.REMOTETESTDOWN }}
 
 jobs:
   build-and-test-autotools:

.github/workflows/run_tests_win_mingw.yml

@@ -6,6 +6,10 @@
 
 name: Run MSYS2, MinGW64-based Tests (Not Visual Studio)
 
+env:
+  CPPFLAGS: "-D_BSD_SOURCE"
+  REMOTETESTDOWN: ${{ vars.REMOTETESTDOWN }}
+
 on: [pull_request,workflow_dispatch]
 
 concurrency: