mxqd: Give reaper thread fixed name

Give reaper thread a fixed name "mxqd reaper". This can be used by a
restarting mxqd to verify that a runnings jobs pid in the database still
identifies a reaper process and hasn't been reused after systen reboot
or pid wrap and still identifies the reaper process.

The checks can be implemented only after the rolling upgrade adding this
commit is completed and all jobs running during the upgrade have
finished.

mxqd.c

@@ -1184,6 +1184,8 @@ int user_process(struct mxq_group_list *glist, struct mxq_job *job)
     return res;
 }
 
+static const char REAPER_PNAME[] = "mxqd reaper";
+
 int reaper_process(struct mxq_server *server,struct mxq_group_list *glist, struct mxq_job *job) {
     pid_t pid;
     struct rusage rusage;
@@ -1201,6 +1203,12 @@ int reaper_process(struct mxq_server *server,struct mxq_group_list *glist, struc
 
     group = &glist->group;
 
+    res = prctl(PR_SET_NAME, REAPER_PNAME, NULL, NULL, NULL);
+        if (res < 0) {
+        mx_log_warning("reaper_process set name: %m");
+        return res;
+    }
+
     res = setsid();
     if (res < 0) {
         mx_log_warning("reaper_process setsid: %m");