fix a couple of small regressions (#1004)

- openssl: fix potentially missing `ERR_*` constants by including
  `openssl/err.h`. This could happen with recent version of Xcode
  or when building against OpenSSL built with the `OPENSSL_NO_ENGINE`
  option.

  Regression from 097c8f0dae (#789)

- channel: fix an issue that would corrupt the data stream when
  attempting to initialize the agent in non-blocking mode, as it is
  necessary to propagate the `EAGAIN` signal upstream when the transport
  returns `EAGAIN`.

  Regression from bc4e619e76 (#752)

- packet: the current code does not set the state machine upon reaching
  this point which means that if the code is suspended due to the
  transport returning an `EAGAIN`, this will re-initialize the structure
  every time.

  The issue is that this keeps assigning a new channel-id downstream,
  which does not match the initial channel-id that is initially
  generated, causing a lookup later to fail as there is no matching
  channel.

  Regression from bc4e619e76 (#752)

src/channel.c

@@ -1165,6 +1165,7 @@ static int channel_request_auth_agent(LIBSSH2_CHANNEL *channel,
         if(rc == LIBSSH2_ERROR_EAGAIN) {
             _libssh2_error(session, rc,
                            "Would block sending auth-agent request");
+            return rc;
         }
         else if(rc) {
             channel->req_auth_agent_state = libssh2_NB_state_idle;

src/openssl.h

@@ -89,6 +89,7 @@
 #ifndef OPENSSL_NO_MD5
 #include <openssl/md5.h>
 #endif
+#include <openssl/err.h>
 #include <openssl/evp.h>
 #include <openssl/hmac.h>
 #include <openssl/bn.h>

src/packet.c

@@ -1208,6 +1208,7 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
                        sizeof(session->packAdd_authagent_state));
 
               libssh2_packet_add_jump_authagent:
+                session->packAdd_state = libssh2_NB_state_jumpauthagent;
                 rc = packet_authagent_open(session, data, datalen,
                                            &session->packAdd_authagent_state);
             }