security_context_v1: set CLOEXEC for client FDs

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/work_items/3949

util/fd.c

@@ -0,0 +1,22 @@
+#include <fcntl.h>
+#include <wlr/util/log.h>
+
+#include "util/fd.h"
+
+bool set_cloexec(int fd, bool cloexec) {
+	int flags = fcntl(fd, F_GETFD);
+	if (flags == -1) {
+		wlr_log_errno(WLR_ERROR, "fcntl failed");
+		return false;
+	}
+	if (cloexec) {
+		flags = flags | FD_CLOEXEC;
+	} else {
+		flags = flags & ~FD_CLOEXEC;
+	}
+	if (fcntl(fd, F_SETFD, flags) == -1) {
+		wlr_log_errno(WLR_ERROR, "fcntl failed");
+		return false;
+	}
+	return true;
+}

util/meson.build

@@ -3,6 +3,7 @@ wlr_files += files(
 	'array.c',
 	'box.c',
 	'env.c',
+	'fd.c',
 	'global.c',
 	'log.c',
 	'matrix.c',