Acme adds a space after Look at the end of window tags.  Therefore, the
space before Send is not needed.  The space after Send is for consistency
and convenience.

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.

An experiment. Let's see if it's any good.
Also document the Mac conventions in devdraw(3).

Fixes #376.

The issue manifests in fork: POSIX fork mandates that a
fork'd process is created with a single thread.  If a
multithreaded program forks, and some thread was in
malloc() when the fork() happened, then in the child
the lock will be held but there will be no thread to
release it.

We assume the system malloc() must already know how to
deal with this and is thread-safe, but it won't know about
our custom spinlock.  Judging that this is no longer
necessary (the lock code was added 15 years ago) we remove
it.

Signed-off-by: Dan Cross <cross@gajendra.net>

Remote whitespace at the ends of lines.
Remove blank lines from the ends of files.
Change modes on source files so that they
are not executable.

Signed-off-by: Dan Cross <cross@gajendra.net>

Add a menu item which functions similar to acme's `Look` command.

This is copied from 9front. See:
https://code.9front.org/hg/plan9front/rev/1f1596dbca51
https://code.9front.org/hg/plan9front/rev/d2de1d2f7b48