Tolerate incomplete socket send/recv

[Josh Stone]

It's not guaranteed that a recv will get an atomic message. With "ssh-pageant -d" and looping "ssh-add -l" requests, I eventually see:

ssh-pageant: recv(0) = 4 (expected 5)

Currently when an incomplete message is detected, that connection is closed, though ssh-pageant will continue listening for new connections. It would be better to have per-connection buffers, so it can tolerate recv'ing partial chunks.

I'm not sure if send will realistically ever be partial, but it might as well be careful.

[Josh Stone]

Allocate a buffer for each connection

Using distinct buffers allows resuming a partial send/recv. Just update
the pointer completed so far in that buffer and go back into the select
loop.

Closed by 47a34c7

[teratorn]

Yeah, sends and receives can always be partial, and must never be relied on to be otherwise.

This commit does seem to fix my issues. Thanks!