selinux: new helper - selinux_add_opt()

the guts of the loop in selinux_parse_opts_str() - takes one
(already recognized) option and adds it to growing selinux_mnt_opts.

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>