Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

Remove $FreeBSD$: one-line sh pattern

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

Convert to LIBADD
Reduce overlinking

Make WARNS=6 the default for libexec/.

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

Make WARNS=6 the default for libexec/.

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.

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Let bsd.prog.mk set SRCS and MAN to their default values.

Lock down with WFORMAT?=1, with overrides in the subdirectories which
are not yet warning-clean. Tested on i386 and alpha.

- Backout botched attempt to intoduce MANSECT feature.
- MAN[1-9] -> MAN.

Set the default manual section for libexec/ to 8.

pppoed(8) - a server to accept PPPoE connections