direct_tcpip: Fixed channel write
direct_tcpip: Fixed channel write

There were 3 bugs in this loop:
1) Started from beginning after partial writes
2) Aborted when 0 bytes were sent
3) Ignored LIBSSH2_ERROR_EAGAIN

See also:
https://trac.libssh2.org/ticket/281
https://trac.libssh2.org/ticket/293

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2