removed libssh2_error()'s forth argument

libssh2_error() no longer allocates a string and only accepts a const
error string. I also made a lot of functions use the construct of
return libssh2_error(...) instead of having one call to
libssh2_error() and then a separate return call. In several of those
cases I then also changed the former -1 return code to a more
detailed one - something that I think will not change behaviors
anywhere but it's worth keeping an eye open for any such.

src/agent.c

@@ -619,7 +619,7 @@ libssh2_agent_init(LIBSSH2_SESSION *session)
     agent = LIBSSH2_ALLOC(session, sizeof *agent);
     if (!agent) {
         libssh2_error(session, LIBSSH2_ERROR_ALLOC,
-                      "Unable to allocate space for agent connection", 0);
+                      "Unable to allocate space for agent connection");
         return NULL;
     }
     memset(agent, 0, sizeof *agent);