refactor(mdns): consolidate platform-specific syscall files

- Rename `syscall_linux.go` to `syscall.go` with build constraints for non-BSD and non-Windows platforms.
- Merge `syscall_darwin.go` into `syscall_bsd.go` and adjust build constraints for BSD platforms (Darwin, FreeBSD, OpenBSD).
- Remove redundant `syscall_freebsd.go`.
- Add build constraints to `syscall_windows.go` for Windows platform.

pkg/mdns/syscall.go

@@ -1,3 +1,5 @@
+//go:build !(darwin || ios || freebsd || openbsd || netbsd || dragonfly || windows)
+
 package mdns
 
 import (

pkg/mdns/syscall_bsd.go

@@ -1,3 +1,5 @@
+//go:build darwin || ios || freebsd || openbsd || netbsd || dragonfly
+
 package mdns
 
 import (

pkg/mdns/syscall_freebsd.go

@@ -1,24 +0,0 @@
-package mdns
-
-import (
-	"syscall"
-)
-
-func SetsockoptInt(fd uintptr, level, opt int, value int) (err error) {
-	// change SO_REUSEADDR and REUSEPORT flags simultaneously for BSD-like OS
-	// https://github.com/AlexxIT/go2rtc/issues/626
-	// https://stackoverflow.com/questions/14388706/how-do-so-reuseaddr-and-so-reuseport-differ/14388707
-	if opt == syscall.SO_REUSEADDR {
-		if err = syscall.SetsockoptInt(int(fd), level, opt, value); err != nil {
-			return
-		}
-
-		opt = syscall.SO_REUSEPORT
-	}
-
-	return syscall.SetsockoptInt(int(fd), level, opt, value)
-}
-
-func SetsockoptIPMreq(fd uintptr, level, opt int, mreq *syscall.IPMreq) (err error) {
-	return syscall.SetsockoptIPMreq(int(fd), level, opt, mreq)
-}

pkg/mdns/syscall_windows.go

@@ -1,3 +1,5 @@
+//go:build windows
+
 package mdns
 
 import "syscall"