B's usage message redirects its stderr to stdout instead of vice versa.
Fix this.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>

Give `-Wformat=0` on illumos/Solaris so that the build does not
complain about `execlp` not having a sentinel (it's nil).  Also
fix an integer size issue when casting to a void pointer.

Use `extern` where appropriate so that we can remove `-fcommon`
on macOS and others.  On macOS, `-fcommon` generates a number of
linker warnings.

I've tested building on macOS, Linux, FreeBSD, NetBSD, OpenBSD
and illumos.  I am not in a position to test on AIX or other
more esoteric platforms, however.

Introduce a new variable set in $PLAN9/config:
$egrep, which is conditionally set to either
`egrep` or `grep -E`, based on platform, and use
this when we want to invoke `egrep`.

This works around the endless warnings from GNU
grep stating that `egrep` is deprecated in favor
of `grep -E`, when run on e.g. Linux.

This tells bin/man to set the register L to very high to avoid pagination
and updates tmac/tmac.an to use that value, if it's set, to set the page
length. This is per Plan 9's rc/bin/man and sys/lib/tmac/tmac.an.