cifs.upcall: try getting a "cifs/" principal and fall back to "host/"

cifs.upcall: try getting a "cifs/" principal and fall back to "host/"

cifs.upcall takes a "-c" flag that tells the upcall to get a principal
in the form of "cifs/hostname.example.com@REALM" instead of
"host/hostname.example.com@REALM". This has turned out to be a source of
great confusion for users.

Instead of requiring this flag, have the upcall try to get a "cifs/"
principal first. If that fails, fall back to getting a "host/"
principal.

Signed-off-by: Jeff Layton <jlayton@redhat.com>