fstrm_capture: Ignore SIGPIPE

Some Frame Streams implementations send FSTRM_CONTROL_STOP to
fstrm_capture without waiting to receive fstrm_capture's subsequent
FSTRM_CONTROL_FINISH. This can cause fstrm_capture to abruptly exit due
to an unhandled SIGPIPE signal, for instance if fstrm_capture is not
running under systemd where IgnoreSIGPIPE= defaults to true.

This behavior was reported by a user on the unbound-users mailing list:
https://lists.nlnetlabs.nl/pipermail/unbound-users/2020-December/007141.html

This commit makes fstrm_capture ignore SIGPIPE, which will cause the
interrupted connections to generate an EPIPE instead, which will be
logged by cb_close_conn(). Perhaps it's also worthwhile to suppress the
logging of EPIPE (or lower the verbosity from CONN_CRITICAL) since it
now occurs normally in recent versions of Unbound, but I haven't done so
in this commit.

This matches the behavior in libfstrm which ignores SIGPIPE in its I/O
thread.

src/fstrm_capture.c

@@ -1201,6 +1201,10 @@ setup_signals(void)
 	if (sigaction(SIGINT, &sa, NULL) != 0)
 		return false;
 
+	sa.sa_handler = SIG_IGN;
+	if (sigaction(SIGPIPE, &sa, NULL) != 0)
+		return false;
+
 	/* Success. */
 	return true;
 }