tmpfiles: add '=' action modifier.

Add the '=' action modifier that instructs tmpfiles.d to check the file
type of a path and remove objects that do not match before trying to
open or create the path.

BUG=chromium:1186405
TEST=./test/test-systemd-tmpfiles.py "$(which systemd-tmpfiles)"

Change-Id: If807dc0db427393e9e0047aba640d0d114897c26

src/basic/fs-util.c

@@ -733,7 +733,7 @@ int inotify_add_watch_and_warn(int fd, const char *pathname, uint32_t mask) {
         return wd;
 }
 
-static bool unsafe_transition(const struct stat *a, const struct stat *b) {
+bool unsafe_transition(const struct stat *a, const struct stat *b) {
         /* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to
          * privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files
          * making us believe we read something safe even though it isn't safe in the specific context we open it in. */