New function log_clock.

* common/logging.c (log_clock): New.
* g10/gpg.c (set_debug): Print clock debug flag.
* g10/options.h (DBG_CLOCK_VALUE, DBG_CLOCK): New.

To actually use log_clock you need to enable the code in
logginc.c:log_check() and link against librt. --debug 4096 may then
be used to enable it at runtime.