Define and use LIBSSH2_INVALID_SOCKET instead of INVALID_SOCKET

INVALID_SOCKET is a special value in Windows representing a
non-valid socket identifier. We were #defining this to -1 on
non-Windows platforms, causing unneccessary namespace pollution.
Let's have our own identifier instead.

Thanks to Matt Lawson for pointing this out.

include/libssh2.h

@@ -135,9 +135,10 @@ typedef long long libssh2_int64_t;
 
 #ifdef WIN32
 typedef SOCKET libssh2_socket_t;
+#define LIBSSH2_INVALID_SOCKET INVALID_SOCKET
 #else /* !WIN32 */
 typedef int libssh2_socket_t;
-#define INVALID_SOCKET -1
+#define LIBSSH2_INVALID_SOCKET -1
 #endif /* WIN32 */
 
 /* Part of every banner, user specified or not */

src/agent.c

@@ -338,7 +338,7 @@ agent_transact_pageant(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 static int
 agent_disconnect_pageant(LIBSSH2_AGENT *agent)
 {
-    agent->fd = INVALID_SOCKET;
+    agent->fd = LIBSSH2_INVALID_SOCKET;
     return 0;
 }
 
@@ -771,7 +771,7 @@ libssh2_agent_userauth(LIBSSH2_AGENT *agent,
 LIBSSH2_API int
 libssh2_agent_disconnect(LIBSSH2_AGENT *agent)
 {
-    if (agent->ops && agent->fd != INVALID_SOCKET)
+    if (agent->ops && agent->fd != LIBSSH2_INVALID_SOCKET)
         return agent->ops->disconnect(agent);
     return 0;
 }
@@ -785,7 +785,7 @@ libssh2_agent_disconnect(LIBSSH2_AGENT *agent)
 LIBSSH2_API void
 libssh2_agent_free(LIBSSH2_AGENT *agent) {
     /* Allow connection freeing when the socket has lost its connection */
-    if (agent->fd != INVALID_SOCKET) {
+    if (agent->fd != LIBSSH2_INVALID_SOCKET) {
         libssh2_agent_disconnect(agent);
     }
     agent_free_identities(agent);

src/session.c

@@ -675,7 +675,7 @@ session_startup(LIBSSH2_SESSION *session, libssh2_socket_t sock)
     if (session->startup_state == libssh2_NB_state_idle) {
         _libssh2_debug(session, LIBSSH2_TRACE_TRANS,
                        "session_startup for socket %d", sock);
-        if (INVALID_SOCKET == sock) {
+        if (LIBSSH2_INVALID_SOCKET == sock) {
             /* Did we forget something? */
             return _libssh2_error(session, LIBSSH2_ERROR_BAD_SOCKET,
                                   "Bad socket provided");