libceph: deprecate [no]cephx_require_signatures options

libceph: deprecate [no]cephx_require_signatures options

These options were introduced in 3.19 with support for message signing
and are rather useless, as explained in commit a51983e4dd2d ("libceph:
add nocephx_sign_messages option").  Deprecate them.

In case there is someone out there with a cluster that lacks support
for MSG_AUTH feature (very unlikely but has to be considered since we
haven't formally raised the bar from argonaut to bobtail yet), make
nocephx_sign_messages also waive MSG_AUTH requirement.  This is probably
how it should have been done in the first place -- if we aren't going
to sign, requiring the signing feature makes no sense.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>