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.

CHANGELOG.md

@@ -72,6 +72,8 @@
   “en_US.UTF-8” if started with a non-UTF8 locale. If this fails, foot
   will start, but only to display a window with an error (user’s shell
   is not executed).
+* `gettimeofday()` has been replaced with `clock_gettime()`, due to it being
+  marked as obsolete by POSIX.
 
 
 ### Deprecated