Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

pw: respect path specified using -V when writing pw.conf, and -C is not
explicitly specified. -V path is already used to determine which file
to read default values from, so it's only logical to write them to the
same file.

PR: 231653
Reviewed by: eugen, bapt
Approved by: re (gjb), kib (mentor)
Differential Revision: https://reviews.freebsd.org/D17566

Remove some unneeded headers

Found by 'include-what-you-use'

Cleanup a bit includes

Rewrite parsing subcommands arguments of pw(8)

Now each subcommands checks its arguments in a dedicated functions.

This helps improving input validation, code readability/maintainability
While here:
- Add a -y option to pw userdel/usermod so it can maintain NIS servers if
nispasswd is not defined in pw.conf(5)
- Allow pw -r <rootdir> to remove directory with userdel -r
- Fix bug when renaming a user which was not renaming the user name it groups
it is a member of.
- Only parse pw.conf(5) when needed.