set the close-on-exec flag on all file descriptors

Added the "fd_util" library, which attempts to use the new thread-safe
Linux system calls pipe2(), accept4() and the options O_CLOEXEC,
SOCK_CLOEXEC.  Without these, it falls back to FD_CLOEXEC, which is
not thread safe.

This is particularly important for the "pipe" output plugin (and
others, such as JACK/PulseAudio), because we were heavily leaking file
descriptors to child processes.

src/socket_util.c

@@ -18,6 +18,7 @@
  */
 
 #include "socket_util.h"
+#include "fd_util.h"
 #include "config.h"
 
 #include <errno.h>
@@ -102,7 +103,7 @@ socket_bind_listen(int domain, int type, int protocol,
 	int passcred = 1;
 #endif
 
-	fd = socket(domain, type, protocol);
+	fd = socket_cloexec(domain, type, protocol);
 	if (fd < 0) {
 		g_set_error(error, listen_quark(), errno,
 			    "Failed to create socket: %s", g_strerror(errno));