Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

Update GNU_DIFF knob descriptions

After r317209 the WITH_/WITHOUT_GNU_DIFF knob controls only diff3;
diff is always BSD diff.

MFC after: 1 week

Add some non-default src.conf(5) knob descriptions

Some WITH_/WITHOUT_ defaults will likey change in the future (e.g. as we
migrate to copyfree base system components).

Add non-default descriptions for the benefit of WIP branches.