smbd: bring back "smb3 unix extensions" option

smbd: bring back "smb3 unix extensions" option

This basically reverts commit b3cae8dcf192f65031f143e5bb9135c895611d98
with a few important differences:

* SMB3 UNIX extensions are always built, but disabled by default at runtime.

* They are globally enabled in the fileserver test environment.

* It's now a per-share option, so admins can selectively disable them
  on a per-share basis. This allows clients to detect early that a share
  doesn't support user mount requested POSIX and fail appropiately, passing
  the failure to the requesting application (mount command).

Signed-off-by: Ralph Boehme <slow@samba.org>