Move chown to fpm_unix_set_socket_premissions()

For consistency, with fpm_unix_resolve_socket_premissions.
Compute + Use in the same source file.
To make easier future enhancement.

Also check chdir output to fix a build warning.
Merge branch 'PHP-5.6'

* PHP-5.6:
  Move chown to fpm_unix_set_socket_premissions()