tds_connect does too much

tds_connect() connects to the server and logs in.  I plan to change it to
only connect.  tds_connect() will return immediately from
tds_open_socket().  

The rest of the logic, the part that sends the login packet, will be moved
to a new function.  But what to call it?  Our namespace is so crowded!  
Ideally, it would be called tds_login(), but tds_login is a data
structure.  Next best is tds_send_login(), but that's already taken for
sending 4.2/5.0 login packets.  

Best is to rename tds_send_login() to tds5_send_login(), and call the new
function tds_send_login().  

This will make it easier to distinguish between network-level errors --
DNS lookup errors, server not listening -- from login failures.  

It will also make it possible to change tsql to make -U and -P optional,
so the user could be prompted for a username after the connection was
made.  You'll be able to see the difference between network and
authentication failures:

	$ tsql -S servername
	locale is "C"
	locale charset is "646"
	connected, please log in
	username: superman
	password: ********
	1>

or

	$ tsql -S servername
	locale is "C"
	locale charset is "646"
	Msg 20008, Level 9, State -1, Server OpenClient, Line -1
	Unable to open socket
	There was a problem connecting to the server

Eventually I'd like to see functionality like this: 

	$ tsql 
	locale is "C"
	locale charset is "646"
	servername: elixer
	elixer not found in /home/username/.freetds.conf
	elixer not found in /etc/freetds.conf
	/etc/interfaces not found
	elixer.example.com has address 10.1.1.14
	connecting to port 4000 with TDS version 5.0
	Msg 20008, Level 9, State -1, Server OpenClient, Line -1
	Unable to open socket
	There was a problem connecting to the server

But that requires rewriting the configuration file parser....

Comments welcome. 

--jkl