Support external recv and send callbacks

[Joseph Southwell <jsouthwell@xxxxxxxxxxxxx>]

This patch allows you to replace the calls to send and recv with alternate callbacks. I am using this because I am running an event driven network stack with stackfull coroutines in c++. This makes the whole thing appear blocking to libssh while still sharing threads efficiently. Socket closure is non blocking so I did not add a callback for that but if somebody wanted to extend this to work with something that isn’t really sockets they would need to add a close callback as well since ssh_disconnect calls close on the socket fd.

Attachment: 0001-got-external-io-working-in-libssh-client.patch
Description: Binary data