merge r3473, r3474, r3385, r3386, r3537: signal processing stability: *) use sys_errlist[] in signal handler instead of non Async-Signal-Safe strerror_r() *) do not update time in the timer signal handler, since localtime_r() is not Async-Signal-Safe function *) use previously cached GMT offset value to update time from a signal handler *) change ngx_time_update() interface since there are no notification methods those return time *) introduce ngx_time_sigsafe_update() to update the error log time only *) change ngx_time_update() interface