Ignore EINTR errors from main poll() loop

When trying to connect the strace tool to the daemon's main process, the
daemon would die because strace interrupted the poll() causing it to
return EINTR.

It should be safe to ignore EINTR errors from poll in this case. Either
the received signal will be otherwise ignored by the process, or it will
cause it to terminate.

postsrsd.c

     char keybuf[1024], *key;
 
     if (poll(fds, 2, 1000) < 0) {
+      if (errno == EINTR)
+        continue;
       if (daemonize)
         syslog (LOG_MAIL | LOG_ERR, "Poll failure: %s", strerror(errno));
       else