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.

misc.h

@@ -2,5 +2,8 @@
 
 #include <stdbool.h>
 #include <wchar.h>
+#include <time.h>
 
 bool isword(wchar_t wc, bool spaces_only, const wchar_t *delimiters);
+
+void timespec_sub(const struct timespec *a, const struct timespec *b, struct timespec *res);