Replace localtime() with localtime_r()

Since the result of localtime() is stored in a statically allocated
structure, data was overwritten when a context switch occurred during
(or shortly after) the execution of localtime(), potentially resulting
in critical data corruption. BUG#7 and BUG#8 are likely related.

This patch converts all usages of localtime() with localtime_r(), which
is thread-safe.

Reported-by: Baptiste Jonglez <baptiste@jonglez.org>
Reported-by: Erik Saule <esaule@bmi.osu.edu>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/pcal.c

@@ -64,7 +64,7 @@ foreach_date_dump(const long date_end, struct rpt *rpt, llist_t * exc,
   time_t t;
 
   t = item_first_date;
-  lt = *localtime(&t);
+  localtime_r(&t, &lt);
   lt.tm_hour = lt.tm_min = lt.tm_sec = 0;
   lt.tm_isdst = -1;
   date = mktime(&lt);