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.h

@@ -94,6 +94,8 @@ enum {
         CHASE_WARN        = 1 << 7, /* Emit an appropriate warning when an error is encountered */
 };
 
+bool unsafe_transition(const struct stat *a, const struct stat *b);
+
 /* How many iterations to execute before returning -ELOOP */
 #define CHASE_SYMLINKS_MAX 32