Handle sigbus like sigsegv

For the purpose of our signal handler (and, in particular, the stack
overflow detection heuristic) they are identical.

Fixes #2695 (as confirmed by @lkarsten)

bin/varnishd/cache/cache_main.c

@@ -243,9 +243,10 @@ child_signal_handler(int s, siginfo_t *si, void *c)
 	memset(&sa, 0, sizeof sa);
 	sa.sa_handler = SIG_DFL;
 	(void)sigaction(SIGSEGV, &sa, NULL);
+	(void)sigaction(SIGBUS, &sa, NULL);
 	(void)sigaction(SIGABRT, &sa, NULL);
 
-	while (s == SIGSEGV) {
+	while (s == SIGSEGV || s == SIGBUS) {
 		req = THR_GetRequest();
 		if (req == NULL || req->wrk == NULL)
 			break;

bin/varnishtest/tests/c00057.vtc

@@ -76,7 +76,7 @@ client c2 -connect ${v2_sock} {
 	expect_close
 } -run
 
-varnish v2 -cliexpect "Segmentation [fF]ault" "panic.show"
+varnish v2 -cliexpect "[bB]us error|Segmentation [fF]ault" "panic.show"
 
 varnish v2 -cliok "panic.clear"