PR19525: always use a SIGUSR2 handler in stapio

PR19525: always use a SIGUSR2 handler in stapio

It was conditionally ignored by commit 62d2a73ee995, to avoid confusing
error messages about file rotation when there's no output file.

But bulk-mode close_relayfs() still needs the signal to cause EINTR on
the waiting threads, so SIG_IGN is too much.  Now we just return from
the handler immediately if there's no output file to rotate.

Also add a SIGUSR2 test for the original error and with bulk mode.