GUACAMOLE-2107: Fix the error handling within the guac_tcp_connect() function.

[necouchman]

This pull request makes a couple of changes within the guac_tcp_connect() function that correct issues that cause the Wake-on-LAN functionality to break, and probably also break other protocols that rely on this function:

• The function returned either the socket that was connected, or various error codes from other calls to functions like connect(), select(), etc. The callers of guac_tcp_connect() rely on the return value being either a socket or a negative integer upon failure, so I've updated many of the instances of the fd variable that might get returned to be -1 in the instances where it would be returned.
• The simple call to the select() function for waiting for a timeout when attempting to connect to a non-blocking socket requires a bit more checking to see if it has succeeded or failed, which involves querying the socket with getsockopt() and looking at the error message.

[mike-jumper]

Minor style nitpick, otherwise LGTM.