util-lib: rework /tmp and /var/tmp handling code

Beef up the existing var_tmp() call, rename it to var_tmp_dir() and add a
matching tmp_dir() call (the former looks for the place for /var/tmp, the
latter for /tmp).

Both calls check $TMPDIR, $TEMP, $TMP, following the algorithm Python3 uses.
All dirs are validated before use. secure_getenv() is used in order to limite
exposure in suid binaries.

This also ports a couple of users over to these new APIs.

The var_tmp() return parameter is changed from an allocated buffer the caller
will own to a const string either pointing into environ[], or into a static
const buffer. Given that environ[] is mostly considered constant (and this is
exposed in the very well-known getenv() call), this should be OK behaviour and
allows us to avoid memory allocations in most cases.

Note that $TMPDIR and friends override both /var/tmp and /tmp usage if set.

src/machine/machined-dbus.c

@@ -954,7 +954,7 @@ static int method_clean_pool(sd_bus_message *message, void *userdata, sd_bus_err
         /* Create a temporary file we can dump information about deleted images into. We use a temporary file for this
          * instead of a pipe or so, since this might grow quit large in theory and we don't want to process this
          * continuously */
-        result_fd = open_tmpfile_unlinkable("/tmp/", O_RDWR|O_CLOEXEC);
+        result_fd = open_tmpfile_unlinkable(NULL, O_RDWR|O_CLOEXEC);
         if (result_fd < 0)
                 return -errno;