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.

Fixes:

% sam -d
 -.
w foo
foo: (new file) #0
w foo
?warning: write might change good version of `foo'

Usually r->nused < r->nalloc and the read is in bounds.
But it could in theory be right on the line and reading
past the end of the allocation.

Make it safe but preserve as much of the old semantics
as possible. This use of rterm appears to be only for
optimization purposes so the result does not matter
for correctness.

The exit code here is ignored anyway.

For whatever reason all three of these programs
contain switches like:

	switch(x) {
	case 1:
		if(cond)
	case 2:
			f();
	}

Like Duff's device, this is legal C but more obscure
than it really needs to be.

This commit assumes those are intended as written
and simply writes them more clearly. I did consider
that maybe they are mistakes, but in the case of sam/regexp.c,
my rewrite in this commit matches the acme/regx.c that
has been in plan9port since I added acme in 2003.
(I didn't bother to dig up the old Plan 9 releases.)

Assuming acme/regx.c has been correct for the past
two decades, this commit should be correct too.

This should make the AIX build finally work.

Fixes #400.

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>