acme, sam, samterm: remove weird switch usage

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.

src/cmd/samterm/flayer.c

@@ -169,8 +169,8 @@ newvisibilities(int redraw)
 			break;
 
 		case V(Some, Some):
-			if(l->f.b==0 && redraw)
 		case V(None, Some):
+			if(ov == None || (l->f.b==0 && redraw))
 				flprepare(l);
 			if(l->f.b && redraw){
 				flrefresh(l, l->entire, 0);