string-util: add strstrafter()

strstrafter() is like strstr() but returns a pointer to the first
character *after* the found substring, not on the substring itself.
Quite often this is what we actually want.

Inspired by #27267 I think it makes sense to add a helper for this,
to avoid the potentially fragile manual pointer increment afterwards.

src/basic/mountpoint-util.c

@@ -559,12 +559,12 @@ int dev_is_devtmpfs(void) {
                 if (mid != mount_id)
                         continue;
 
-                e = strstr(line, " - ");
+                e = strstrafter(line, " - ");
                 if (!e)
                         continue;
 
                 /* accept any name that starts with the currently expected type */
-                if (startswith(e + 3, "devtmpfs"))
+                if (startswith(e, "devtmpfs"))
                         return true;
         }