mount.cifs: deprecate the DOMAIN/username%password username syntax

mount.cifs: deprecate the DOMAIN/username%password username syntax

mount.cifs has in the past allowed users to specify a username using
the above syntax, which would populate the domain and password fields
with the different pieces.

Unfortunately, there are cases where it is legit to have a '/' in a
username. krb5 SPNs generally contain a '/' and we have no clear way
to distinguish between the two.

I don't see any real value in keeping that syntax allowed. It's no
easier than specifying "pass=" and "domain=" on the command line. Ditto
for credential files.

Begin the transition away from that syntax by adding a warning message
that support for it will be removed in 5.9.

Signed-off-by: Jeff Layton <jlayton@samba.org>