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/load-fragment.c

@@ -3620,10 +3620,12 @@ static int load_from_path(Unit *u, const char *path) {
         if (fstat(fileno(f), &st) < 0)
                 return -errno;
 
-        if (null_or_empty(&st))
+        if (null_or_empty(&st)) {
                 u->load_state = UNIT_MASKED;
-        else {
+                u->fragment_mtime = 0;
+        } else {
                 u->load_state = UNIT_LOADED;
+                u->fragment_mtime = timespec_load(&st.st_mtim);
 
                 /* Now, parse the file contents */
                 r = config_parse(u->id, filename, f,
@@ -3638,8 +3640,6 @@ static int load_from_path(Unit *u, const char *path) {
         u->fragment_path = filename;
         filename = NULL;
 
-        u->fragment_mtime = timespec_load(&st.st_mtim);
-
         if (u->source_path) {
                 if (stat(u->source_path, &st) >= 0)
                         u->source_mtime = timespec_load(&st.st_mtim);

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;