mxqd: Restore SIGPIPE to default for job

The mysql client library sets SIGPIPE to SIG_IGN which gets inherited by
the user processes:

    root@sigchld:~# cat /proc/27900/status |grep Sig
    SigQ:   0/30656
    SigPnd: 0000000000000000
    SigBlk: 0000000000000000
    SigIgn: 0000000000001000
    SigCgt: 0000000000000000

This makes a difference for things like

     CMD | head -1

because CMD is not killed by SIGPIPE when `head` is finished. It will get
an error "broken pipe" on the next write and may or may not emit this to
stderr.

This bug was partially caused by #63 (commit ff2f49f (mxqd: Use
synchronous signals)), where we changed our own signal handling to
synchronous mode. While in the previous mode, we unknowingly fixed the
change done by the mysql library, we then just set and reset the blocking
mask, which doesn't restore an ignored signal.

Restore SIGCHLD to SIG_DFL when we initialize the child process for the
user.

With this patch applied, no more signals are blocked for the user
process.

    buczek@theinternet:~/git/mxq (restore-sigpipe)$ cat /proc/17081/status |grep Sig
    SigQ:   1/127301
    SigPnd: 0000000000000000
    SigBlk: 0000000000000000
    SigIgn: 0000000000000000
    SigCgt: 0000000000000000

Fixes: #71

mxqd.c

@@ -719,6 +719,7 @@ static int init_child_process(struct mxq_group_list *glist, struct mxq_job *job)
     group  = &glist->group;
 
     sigprocmask(SIG_UNBLOCK,&all_signals,NULL);
+    signal(SIGPIPE,SIG_DFL);
 
     passwd = getpwuid(group->user_uid);
     if (!passwd) {