Retire the kill(SIGNULL) test in VSM

A bug was uncovered in the VSM code that checks if kill(SIGNULL) would
work as a test of liveness of the master and worker processes. If the user
running the utility has the required permissions to send signal to the
worker process, but not the management process, the code would wrongly
assume it could do kill on both. It would then end up in a connect loop
never succeeding.

Because kill() can not always be successfully run (a common scenario when
the user running varnishlog is not the same UID as the cache processes),
there is a fall back to using fstat to check for Varnish restarts. Since
this fallback is active for most use cases anyways, it was decided to
retire the kill() mechanism rather than to fix it. This way the behaviour
does not change depending on what user the utility is run as.

This change was OK'd by PHK after discussing it on IRC.

bin/varnishtest/tests/u00008.vtc

@@ -37,10 +37,6 @@ process p1 -expect-text 0 0 "VBE.vcl1.s1.req"
 process p1 -expect-text 0 0 "DIAG"
 process p1 -screen_dump
 
-varnish v1 -stop
-process p1 -expect-text 2 1 "Uptime child:   Not Running"
-process p1 -screen_dump
-
 process p1 -write {dek}
 process p1 -expect-text 0 1 "Concurrent connections to backend:"
 process p1 -screen_dump

lib/libvarnishapi/vsm.c

@@ -150,8 +150,6 @@ struct vsm {
 
 	int			attached;
 	double			patience;
-
-	int			couldkill;
 };
 
 /*--------------------------------------------------------------------*/
@@ -357,8 +355,6 @@ VSM_New(void)
 	vd->child->vsm = vd;
 	vd->wdfd = -1;
 	vd->patience = 5;
-	if (getenv("VSM_NOPID") != NULL)
-		vd->couldkill = -1;
 	return (vd);
 }
 
@@ -485,17 +481,13 @@ vsm_vlu_hash(struct vsm *vd, struct vsm_set *vs, const char *line)
 	int i;
 	uintmax_t id1, id2;
 
+	(void)vd;
+
 	i = sscanf(line, "# %ju %ju", &id1, &id2);
 	if (i != 2) {
 		vs->retval |= VSM_MGT_RESTARTED | VSM_MGT_CHANGED;
 		return (0);
 	}
-	if (vd->couldkill >= 0 && !kill(id1, 0)) {
-		vd->couldkill = 1;
-	} else if (vd->couldkill > 0 && errno == ESRCH) {
-		vs->retval |= VSM_MGT_RESTARTED | VSM_MGT_CHANGED;
-		return (0);
-	}
 	vs->retval |= VSM_MGT_RUNNING;
 	if (id1 != vs->id1 || id2 != vs->id2) {
 		vs->retval |= VSM_MGT_RESTARTED | VSM_MGT_CHANGED;
@@ -694,8 +686,7 @@ vsm_refresh_set(struct vsm *vd, struct vsm_set *vs)
 
 	vs->fst.st_size = lseek(vs->fd, 0L, SEEK_CUR);
 
-	if (vd->couldkill < 1 || !kill(vs->id1, 0))
-		vs->retval |= vs->flag_running;
+	vs->retval |= vs->flag_running;
 	return (vs->retval);
 }