client: add pick_my_perms() function for implementers

client: add pick_my_perms() function for implementers

We need to get rid of the internal uid/gid selection and switch to
permission-checking that handles multiple groups -- while also preserving
the client_mount_(uid|gid) config params. So expose a function which
can be called externally that replicates our current uid/gid selection,
but uses our new UserPerm struct.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>