1.0.42.50: workaround a Darwin nanosleep() bug

 Fixes lp#640516.

 It turns out that on Darwin, if a nanosleep() call is interrupted,
 and the signal handler takes longer than the requested sleep time
 was, then the call will return with EINTR and (unsigned)-1 in the
 remaining seconds.

 Since we call nanosleep() again when it returns with EINTR with the
 remaining time, this would cause us to sleep ~136 years...

 So, check that the remainder is not increasing before calling
 nanosleep() again.

 Many, many thanks to Joe Lobraco who reported and diagnosed the
 issue.