time: add runtime sanity check

std::chrono::system_clock.time_since_epoch and time_t(0) are not guaranteed
to use the Unix epoch timestamp, but in practice they almost certainly will.
Any differing behavior will be assumed to be an error, unless certain
platforms prove to consistently deviate, at which point we'll cope with it
by adding offsets.

Do a quick runtime check to verify that
time_t(0) == std::chrono::system_clock's epoch time == unix epoch.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

src/util/time.h

@@ -70,4 +70,7 @@ struct timeval MillisToTimeval(int64_t nTimeout);
  */
 struct timeval MillisToTimeval(std::chrono::milliseconds ms);
 
+/** Sanity check epoch match normal Unix epoch */
+bool ChronoSanityCheck();
+
 #endif // BITCOIN_UTIL_TIME_H