mail/fetchmail: make OPIE and RPA optional, UPDATING info added

Fix a build failure on FreeBSD 14+ with OPIE (now in ports) enabled and
NLS disabled, by adding ${LOCALBASE}/include to the compiler's path.

OPIE and RPA have been made optional because both have significant security
problems. OPIE support was not provided by the port on 14.x until the update
to 6.5.0, when it was briefly re-activated. Because of its security flaws, it
has now been disabled by default on both 13.x and 14.x.

RPA is currently enabled by default, but this will change in a future update.

PR: 282444
Reported by: Helge Oldach