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 a system reboot
or a pid wrap.

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_err("reaper_process set name: %m");
+        return res;
+    }
+
     res = setsid();
     if (res < 0) {
         mx_log_warning("reaper_process setsid: %m");