Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

AUTO_OBJ: For all top-level targets enforce using an OBJDIR.

This will cause an error if the wanted OBJDIR is not writable. Previously it
would cause the files to generate to the source tree. This was too obscure and
things like buildworld really expect a proper OBJDIR layout.

Sponsored by: Dell EMC

Fix top-level targets with read-only OBJDIR.

This also makes it so that top-level build targets do not immediately create
the OBJDIR. Only sub-make targets will do so. This avoids creating object
directories for targets like 'make check-old' or creating unneeded
MACHINE.MACHINE_ARCH directories during 'make tinderbox'.

Reported by: npn, lifanov
Tested by: npn, Mark Millard
Sponsored by: Dell

Move top-level AUTO_OBJ logic to Makefile.sys.inc.

Sponsored by: Dell