[dmctl.cc] Ignore SIGPIPE (we check for errors instead)

On a Raspberry Pi install, daemon-manager is responding to the dmctl connect
so quickly that the first write never even gets out (it always does on my
Mac and x86-64 debian machines). This is the first step to getting the real
error message to print instead of nothing.

Re: dm-9
Re: c5bf7ab5b05c63410d144158adb6c6ddc70bb341

dmctl.cc

@@ -13,6 +13,7 @@
 #include <poll.h>
 #include <stdexcept>
 #include <sys/stat.h>
+#include <signal.h>
 #include "command-sock.h"
 #include "user.h"
 #include "stringutil.h"
@@ -43,6 +44,8 @@ int main(int argc, char **argv)
     if (o.get("help", 'h')) usage(argv[0], EXIT_SUCCESS);
     if (o.bad_args() || o.args.size() > 2) usage(argv[0], EXIT_FAILURE);
 
+    signal(SIGPIPE, SIG_IGN);
+
     int command_socket;
     try {
         struct sockaddr_un addr = command_sock_addr();