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/output/recorder_output_plugin.c

@@ -20,6 +20,7 @@
 #include "output_api.h"
 #include "encoder_plugin.h"
 #include "encoder_list.h"
+#include "fd_util.h"
 
 #include <assert.h>
 #include <sys/types.h>
@@ -156,7 +157,7 @@ recorder_output_open(void *data, struct audio_format *audio_format,
 
 	/* create the output file */
 
-	recorder->fd = creat(recorder->path, 0666);
+	recorder->fd = creat_cloexec(recorder->path, 0666);
 	if (recorder->fd < 0) {
 		g_set_error(error_r, recorder_output_quark(), 0,
 			    "Failed to create '%s': %s",