libast: update /usr/tmp fallback to /var/tmp

To find the temporary files directory to use, the pathtemp()
function (generate a unique path to a temporary file) first checks
$TMPDIR and $TMPPATH, then falls back to /tmp, then to /usr/tmp as
a last resort. But all systems replaced /usr/tmp by /var/tmp
decades ago to allow mounting /usr as read-only, and a /usr/tmp
compatibility symlink is no longer commonly provided.

src/lib/libast/path/pathtemp.c:
- Change TMP2 definition from "/usr/tmp" to "/var/tmp".

src/lib/libast/features/mmap,
src/lib/libast/features/stdio:
- Change "/usr/tmp" to "/var/tmp" in feature tests.

src/lib/libast/features/mmap

@@ -35,7 +35,7 @@ tst	lib_mmap note{ standard mmap interface that works }end execute{
 		/* create data file in a local fs if possible */
 		t = file;
 		if (access(f = "/tmp", 0) == 0 ||
-		    access(f = "/usr/tmp", 0) == 0)
+		    access(f = "/var/tmp", 0) == 0)
 		{
 			while (*t = *f++)
 				t++;