r350143:
Chain Makefile.inc's so default are inherited as expected.

Remove unneeded or duplicate variables.

No functional change.

r350148:
Restore WARNS?=1 here.  I missed that libexec/bootpd/bootpgw existed.

Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL

Just like bin/ and sbin/, I think setting WARNS to the highest value
possible will make it more attractive for people to fix warnings.

- The WARNS variable is set in the Makefile in the directory of the
  application itself, making it more likely that it will be removed out
  of curiosity to see what happens.
- New applications will most likely build with WARNS=6 out of the box,
  because the author would more likely fix the warnings during
  development than lower WARNS.

Unfortunately almost all apps in libexec require a lowered value of
WARNS.

This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.

Boy, I'm glad we're not using sup anymore.  This update would have been
insane otherwise.