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.