mount.cifs: update the mount.cifs manpage

mount.cifs: update the mount.cifs manpage

Add a new section entitled FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS
that attempts to cover information about this topic. Change the uid=
and gid= options to refer to that section. Add new varlistentries for
forceuid, forcegid and dynperm.

Also update the information about how the program behaves when installed
as a setuid binary.

Signed-off-by: Jeff Layton <jlayton@redhat.com>