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/mixer/oss_mixer_plugin.c

@@ -19,6 +19,7 @@
 
 #include "mixer_api.h"
 #include "output_api.h"
+#include "fd_util.h"
 
 #include <glib.h>
 
@@ -122,7 +123,7 @@ oss_mixer_open(struct mixer *data, GError **error_r)
 {
 	struct oss_mixer *om = (struct oss_mixer *) data;
 
-	om->device_fd = open(om->device, O_RDONLY);
+	om->device_fd = open_cloexec(om->device, O_RDONLY);
 	if (om->device_fd < 0) {
 		g_set_error(error_r, oss_mixer_quark(), errno,
 			    "failed to open %s: %s",