change build_compile/write to build_pass1/pass2

[robnagler]

Currently build_compile shouldn't write anything, but many components write something. Instead, pass1 should replace compile and be "arbitrary" as to the boundary.

Change default_command to be three pass:

1. create all build.T objects
2. call component.build_pass1
3. call component.build_pass2
4. finalize the writing

This change would allow components in pass2 to ask questions about all components across all hosts being built. For example, opendkim needs the IPs of all postfix.smart_host clients.