add man pages for passwd and shadow files

The manual pages are greatly inspired by the ones provided by
the shadow-utils upstream project https://github.com/shadow-maint/shadow.

files/usr/share/man/passwd.man

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+NAME
+    passwd - the password file
+
+DESCRIPTION
+    /etc/passwd contains one line for each user account, with seven fields
+    delimited by colons (":").
+
+    These fields are:
+        *   login name
+        *   optional encrypted password
+        *   numerical user ID
+        *   numerical group ID
+        *   user name or comment field
+        *   user home directory
+        *   optional user command interpreter
+
+   If the password field is a lower-case "x", then the encrypted password is
+   actually stored in the shadow file instead; there must be a corresponding
+   line in the /etc/shadow file, or else the user account is invalid.
+
+   The home directory field provides the name of the initial working directory.
+   The login program uses this information to set the value of the $HOME
+   environmental variable.
+
+   The command interpreter field provides the name of the user's command
+   language interpreter, or the name of the initial program to execute.
+   The login program uses this information to set the value of the $SHELL
+   environmental variable.

files/usr/share/man/shadow.man

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+NAME
+    shadow - shadowed password file
+
+DESCRIPTION
+    shadow is a file which contains the password information for the
+    system's accounts.
+
+    This file must not be readable by regular users if password security is to
+    be maintained.
+
+    Each line of this file contains 9 fields, separated by colons (“:”), in the
+    following order:
+
+        *   login name
+        *   encrypted password
+        *   date of last password change
+        *   minimum password age
+        *   maximum password age
+        *   password warning period
+        *   password inactivity period
+        *   account expiration date
+        *   reserved field