replace gettimeofday with clock_gettime

POSIX.1-2008 has marked gettimeofday(2) as obsolete, recommending the
use of clock_gettime(2) instead.

CLOCK_MONOTONIC has been used instead of CLOCK_REALTIME because it is
unaffected by manual changes in the system clock. This makes it better
for our purposes, namely, measuring the difference between two points in
time.

tv_sec has been casted to long in most places since POSIX does not
define the actual type of time_t.

terminal.h

@@ -552,7 +552,7 @@ struct terminal {
         bool urgency;  /* Signal 'urgency' (paint borders red) */
 
         struct {
-            struct timeval last_update;
+            struct timespec last_update;
             bool is_armed;
             int timer_fd;
         } title;