mxqd: Verify names of reaper processes no restart

When the daemon restarts, it has to figure out which of the jobs, the
database shows as running on the sever, are in fact still running and
which are gone.

Currently we only check, whether the process with the pid from the
database still exists.  However, this can give wrong results if the pid
of a job is reused after a system reboot or after a pid wrap. mxqd might
regard an unrelated process as one of its jobs and nanny and kill it.

Update code to only regard a proces as a running mxqd job if its
"comm"-value (/proc/PID/comm) is "mxqd reaper".

mxqd.c

@@ -1187,6 +1187,16 @@ int user_process(struct mxq_group_list *glist, struct mxq_job *job)
 
 static const char REAPER_PNAME[] = "mxqd reaper";
 
+static int is_reaper(pid_t pid) {
+    char comm[16];
+    if (mx_proc_get_comm(pid, comm) == NULL)
+        return 0;
+    if (strcmp(comm, REAPER_PNAME) == 0)
+        return 1;
+    else
+        return 0;
+}
+
 int reaper_process(struct mxq_server *server,struct mxq_group_list *glist, struct mxq_job *job) {
     pid_t pid;
     struct rusage rusage;
@@ -2285,8 +2295,6 @@ static int lost_scan_one(struct mxq_server *server)
 
     struct mxq_job *job;
 
-    int res;
-
     for (ulist = server->users; ulist; ulist = ulist->next) {
         for (glist = ulist->groups; glist; glist = glist->next) {
             for (jlist = glist->jobs; jlist; jlist = jlist->next) {
@@ -2304,13 +2312,9 @@ static int lost_scan_one(struct mxq_server *server)
                     continue;
                 }
 
-                res = kill(job->host_pid, 0);
-                if (res >= 0)
+                if (is_reaper(job->host_pid))
                     continue;
 
-                if (errno != ESRCH)
-                    return -errno;
-
                 if (!fspool_file_exists(server, job->job_id)) {
                     mx_log_warning("pid %u: process is gone. setting job status of job %lu to unknown.",
                                 jlist->job.host_pid,