www/mod_auth_gssapi: Add options for base/ports MIT Kerberos

Allow selection of base MIT on FreeBSD 15+.

Improve port:

• Replace PORTVERSION with DISTVERSION.
• Merge CONFIGURE_ARGS and repalce "+=" with "=" - unnecessary.
• Remove "+" in "PLIST_SUB+=" - unnecessary.
• Fix warnings from portclippy.
• Merge MKDIRs in do-install.

PR: 292474
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>