shred: use gethrxtime instead of time

shred: use gethrxtime instead of time

The gethrxtime function uses a monotonic clock if available on the
system. The effect should be unnoticeable since we print --verbose
information every 5 seconds, but is more correct and we already do the
same in 'dd'.

* src/local.mk (src_shred_LDADD): Add $(GETHRXTIME_LIB).
* src/shred.c (dopass): Use xtime_t instead of time_t. Use gethrxtime
instead of time.