Add configuration option to run the GC on exit

If "auto_gc" is enabled, the garbage collector for note files will be
run on every exit. As this is an experimental feature and may cause data
loss, this is disabled by default.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -219,6 +219,7 @@
 /* General configuration variables. */
 struct conf {
   unsigned   auto_save;
+  unsigned   auto_gc;
   unsigned   periodic_save;
   unsigned   confirm_quit;
   unsigned   confirm_delete;