agent: Use clock or clock_gettime for calibration.

* agent/protect.c (calibrate_get_time): Use clock or clock_gettime.

--

For calibration, clock(3) is better than times(3) among UNIXen.
Tested on NetBSD 7.1 and FreeBSD 11.1, using QEMU.

Thanks to Damien Goutte-Gattat for the information of use of
CLOCKS_PER_SEC;  The old code with times(3) is not 100% correct,
in terms of POSIX.  It should have used sysconf (_SC_CLK_TCK) instead
of CLOCKS_PER_SEC.  CLOCKS_PER_SEC is specifically for clock(3).

GnuPG-bug-id: 3056, 3276, 3472
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
(cherry picked from commit 380bce13d9)

agent/t-protect.c

@@ -322,9 +322,9 @@ test_agent_protect_shared_secret (void)
 int
 main (int argc, char **argv)
 {
-  (void)argc;
   (void)argv;
 
+  opt.verbose = argc - 1;       /* We can do "./t-protect -v -v" */
   gcry_control (GCRYCTL_DISABLE_SECMEM);
 
   test_agent_protect ();