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/basic/fs-util.h

@@ -61,7 +61,8 @@ int mkfifo_atomic(const char *path, mode_t mode);
 
 int get_files_in_directory(const char *path, char ***list);
 
-int var_tmp(char **ret);
+int tmp_dir(const char **ret);
+int var_tmp_dir(const char **ret);
 
 #define INOTIFY_EVENT_MAX (sizeof(struct inotify_event) + NAME_MAX + 1)