man: use configured path for mount and umount binaries in manpages

man: use configured path for mount and umount binaries in manpages

Export the MOUNT_PATH and UMOUNT_PATH variables as XML entities and use them in
the systemctl.1 manpage instead of hardcoding the path in /usr/bin.

Tested:
- Ran ./configure ac_cv_path_MOUNT_PATH=/bin/mount (same for umount) and
  rebuilt the manpages, confirmed that the correct path was in man/systemctl.1
- Rebuilt man/systemd.directives.xml and the man pages derived from it,
  confirmed that the correct paths were there as well.