Feat: Port forwarding closure

[jaywonchung]

Quickly implemented port forwarding closure.

Reference: https://github.com/openssh/openssh-portable/blob/059ed698a47c9af541a49cf754fd09f984ac5a21/PROTOCOL.mux#L189

The request is pretty much the exact same as requesting port forwarding, except that the protocol code is MUX_C_CLOSE_FWD (0x10000007) and the server does not return a MUX_S_REMOTE_PORT.

I realized that the verb "close" aligns better with the protocol doc's language than "cancel". If this gets merged, on the openssh repo, I think I'll rename the APIs (i.e., replace "cancel" with "close").

CC. @NobodyXu

[NobodyXu]

Thank you so much for putting up this PR!

I will a few suggestions, and it looks great to me!

[jaywonchung]

I checked socket closure by checking whether read() returns 0, which seems to mean that the connection is closed. CI is passing in my fork (https://github.com/jaywonchung/openssh-mux-client/actions/runs/10793796561). However, I'm honestly not very knowledgeable in sockets and networking. Please see if this is acceptable.

[NobodyXu]

Thank you!