Path MTU discovery fixes and improvements (#3109)

- Existing option `WIREGUARD_MTU` , if set, disables PMTUD and is used
- New option `PMTUD_ICMP_ADDRESSES=1.1.1.1,8.8.8.8` and `PMTUD_TCP_ADDRESSES=1.1.1.1:443,8.8.8.8:443`
- ICMP PMTUD now targets external-by-default IP addresses
- New TCP PMTUD (binary search only) as a second MTU confirmation and fallback mechanism.
- Force set TCP MSS to MTU - IP header - TCP base header - "magic 20 bytes" 🎆
- Fix #3108

internal/pmtud/tcp/tcp_unix.go

@@ -0,0 +1,28 @@
+//go:build linux || darwin
+
+package tcp
+
+import (
+	"syscall"
+	"time"
+)
+
+// fileDescriptor is a platform-independent type for socket file descriptors.
+type fileDescriptor int
+
+func sendTo(fd fileDescriptor, p []byte, flags int, to syscall.Sockaddr) (err error) {
+	return syscall.Sendto(int(fd), p, flags, to)
+}
+
+func setSocketTimeout(fd fileDescriptor, timeout time.Duration) (err error) {
+	timeval := syscall.NsecToTimeval(timeout.Nanoseconds())
+	return syscall.SetsockoptTimeval(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVTIMEO, &timeval)
+}
+
+func recvFrom(fd fileDescriptor, p []byte, flags int) (n int, from syscall.Sockaddr, err error) {
+	return syscall.Recvfrom(int(fd), p, flags)
+}
+
+func setNonBlock(fd int) error {
+	return syscall.SetNonblock(fd, true)
+}