Mk/Uses/samba.mk: Clean up and define SAMBA_LDB_PORT

• Deprecate the SAMBAPORT, SAMBALIBS, and SAMBAINCLUDES variables and replace them with the more readable and consistent SAMBA_PORT, SAMBA_LIBDIR, and SAMBA_INCLUDEDIR. The new variables follow the conventions of python.mk, ruby.mk, and cargo.mk more and are more future-proof.
• Rearrange the order of IGNORE checks.
• Explicitly mention the ports used samba.mk in a searchable manner (i.e., grepping for net/samba419 is going to return samba.mk).
• Define SAMBA_LDB_PORT, which allows ports to always use the right version of ldb for the Samba selected via SAMBA_DEFAULT.
• Remove old comments about samba.mk becoming more granular once subpackages are committed. Subpackages will not help samba.mk because Samba's bundled dependencies are built as private libraries and are not usable by external software.

PR: 280773
Reviewed by: Xavier Beaudouin <kiwi@oav.net>
Approved by: samba (0mp, Xavier Beaudouin <kiwi@oav.net>)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46322