varname - Name of environment variable to set on the remote
channel instance.
varname_len - Length of passed varname parameter.
value - Value to set varname to.
value_len - Length of value parameter.
Set an environment variable in the remote channel's process space. Note that
this does not make sense for all channel types and may be ignored by the
server despite returning success.
RETURN VALUE
Return 0 on success or negative on failure. It returns
LIBSSH2_ERROR_EAGAIN when it would otherwise block. While
LIBSSH2_ERROR_EAGAIN is a negative number, it isn't really a failure per se.
ERRORS
LIBSSH2_ERROR_ALLOC - An internal memory allocation call failed.
LIBSSH2_ERROR_SOCKET_SEND - Unable to send data on socket.