security/krb5: Support libedit in base

Even though libedit is in base FreeBSD, the krb5 ports still depend
on devel/libedit when the LIBEDIT option is selected. This is because
./configure uses pkgconf to determine if libedit exists, ignoring
libedit in FreeBSD base. This patch adds a new LIBEDIT_BASE option
which enables LIBEDIT (LIBEDIT_BASE) without installing the
devel/libedit port.

The GNU READLINE option will remain the default for now but it is
planned to switch the default to LIBEDIT_BASE at some point. This is
to reduce the dependency on GNU software and to bring it more into
line with the planned MIT KRB5 import into FreeBSD base.

(cherry picked from commit e89f84156a8fcb2f81c1f962845f4456b2f62f63)