net/samba422: Fix USES=cpe and incorporate USES=samba:self

USES=cpe has not been working as it was overwritten by another
assignment of USES later in the file. Fix that by merging those two USES
assignments into one.

Add USES=samba:self, so that we can depend on the directory paths
defined in samba.mk.

PR 290424
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54137