fixits: suggest running --update if there are fixits

* src/fixits.h, src/fixits.c (fixits_empty): New.
* src/complain.c (deprecated_directive): Register the Wdeprecated
fixits only if -Wdeprecated was enabled, so that we don't apply
updates if the user didn't ask for them.
* src/main.c (main): If there were fixits, issue a warning suggesting
running with --update.
Free uniqstrs after the fixits, since the latter use the former.
* tests/headers.at, tests/input.at: Update expectations.

src/fixits.c

@@ -91,6 +91,13 @@ fixits_register (location const *loc, char const* fix)
 }
 
 
+bool
+fixits_empty (void)
+{
+  return !fixits;
+}
+
+
 void
 fixits_run (void)
 {