mount.ceph: fork a child to get info from local configuration

mount.ceph: fork a child to get info from local configuration

When a secret and/or the mon addrs are not specified by the admin,
then mmap a MAP_SHARED buffer and spawn a child process to get that
info. For safety reasons, the child drops all capabilities other than
CAP_DAC_READ_SEARCH (to ensure that it'll be able to read the keyring,
should one be found). To achieve this, we add a new dependency on
libcap-ng.

Add a new C++ file with a single routine that will create a CephContext,
get a list of monitor addresses and scrape the keyring for a secret for
the specified cephx user.

If that info is found, then it is copied to fixed-length buffers in the
MAP_SHARED area and the child exits successfully.

The parent will then vet the returned info and copy it into the
appropriate fields if they are currently blank.

Fixes: https://tracker.ceph.com/issues/16656
Signed-off-by: Jeff Layton <jlayton@redhat.com>