Use SA_RESTART for all signals, including SIGALRM.
The exclusion of SIGALRM dates back to Berkeley days, when Postgres used SIGALRM in only one very short stretch of code. Nowadays, allowing it to interrupt kernel calls doesn't seem like a very good idea, since its use for statement_timeout means SIGALRM could occur anyplace in the code, and there are far too many call sites where we aren't prepared to deal with EINTR failures. When third-party code is taken into consideration, it seems impossible that we ever could be fully EINTR-proof, so better to use SA_RESTART always and deal with the implications of that. One such implication is that we should not assume pg_usleep() will be terminated early by a signal. Therefore, long sleeps should probably be replaced by WaitLatch operations where practical. Back-patch to 9.3 so we can get some beta testing on this change.
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