feat: add ListenerControl callback to Config

[krsna1729]

Summary

Go's net.ListenConfig exposes a Control callback so callers can set arbitrary socket options on the raw fd before bind. gosrt hardcodes its own ListenControl() inside Listen() but provides no way for the caller to hook into the same mechanism.

This PR adds a ListenerControl field to Config, following the same signature as net.ListenConfig.Control. If set, it runs after gosrt's built-in socket options (SO_REUSEADDR, IP_TOS, IP_TTL) but before the socket is bound.

This lets callers like mediamtx set SO_RCVBUF (or any other socket option) without gosrt needing to know about application-specific details:

conf.ListenerControl = func(network, address string, c syscall.RawConn) error {
    return c.Control(func(fd uintptr) {
        syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVBUF, 26214400)
    })
}

Changes

• config.go: Add ListenerControl func(network, address string, c syscall.RawConn) error field to Config
• listen.go: Wrap the net.ListenConfig.Control callback to chain gosrt's built-in ListenControl with the caller's ListenerControl (if set)

Test plan

• All existing tests pass (go test ./...)
• Builds cleanly against latest master
• Zero behavioral change when ListenerControl is nil (default)

[krsna1729]

cc @ioppermann would be nice to have? I plan to send the counterpart app patch to mediamtx once this is merged.