iwatch: ensure adding a new watcher is idempotent

Some subsystems call iwatch_add() without first calling iwatch_del() on
the same path.  E.g., cgroup_config().

Issue #417 but unclear atm. if this is the root cause.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/iwatch.c

@@ -96,6 +96,12 @@ int iwatch_add1(struct iwatch *iw, char *file, uint32_t mask)
 	if (!initialized)
 		return -1;
 
+	iwp = iwatch_find_by_path(iw, file);
+	if (iwp) {
+		dbg("skipping, %s already registered", file);
+		return 0;
+	}
+
 	if (!fexist(file)) {
 		dbg("skipping %s: no such file or directory", file);
 		return 0;