core: treat masked files as "unchanged"

systemctl prints the "unit file changed on disk" warning
for a masked unit. I think it's better to print nothing in that
case.

When a masked unit is loaded, set mtime as 0. When checking
if a unit with mtime of 0 needs reload, check that the mask
is still in place.

src/core/unit.c

@@ -2934,7 +2934,11 @@ static bool fragment_mtime_changed(const char *path, usec_t mtime) {
                 /* What, cannot access this anymore? */
                 return true;
 
-        if (mtime > 0 && timespec_load(&st.st_mtim) != mtime)
+        if (mtime > 0)
+                /* For non-empty files check the mtime */
+                return timespec_load(&st.st_mtim) != mtime;
+        else if (!null_or_empty(&st))
+                /* For masked files check if they are still so */
                 return true;
 
         return false;