linux-v4l2: scandir with alphasort on non-Linux

Sort video device entries with `alphasort` on non-Linux platforms,
as opposed to `versionsort` on Linux.
(`versionsort` is a GNU extension, unavailable on e.g. FreeBSD.)

UI: Fix call to `to_string` on FreeBSD

UI/platform-x11.cpp

@@ -136,7 +136,7 @@ const char *RunOnce::thr_name = "OBS runonce";
 void PIDFileCheck(bool &already_running)
 {
 	std::string tmpfile_name =
-		"/tmp/obs-studio.lock." + to_string(geteuid());
+		"/tmp/obs-studio.lock." + std::to_string(geteuid());
 	int fd = open(tmpfile_name.c_str(), O_RDWR | O_CREAT | O_EXLOCK, 0600);
 	if (fd == -1) {
 		already_running = true;

plugins/linux-v4l2/v4l2-output.c

@@ -185,7 +185,14 @@ static bool virtualcam_start(void *data)
 			return false;
 	}
 
-	n = scandir("/dev", &list, scanfilter, versionsort);
+	n = scandir("/dev", &list, scanfilter,
+#if defined(__linux__)
+		    versionsort
+#else
+		    alphasort
+#endif
+	);
+
 	if (n == -1)
 		return false;