Improved feedback between client and server.

Client is now able to detect when TCP connection is closed and exits
informing user (instead of waiting indefinitely).

Server now signals in-band with two control characters:
\0x3, ASCII End of Text, is sent to separate requests (replacing \0x4 from
previous versions);
\0x4, ASCII End of Transmission, is sent to signal an upcoming closing of
TCP connection, following a fatal error.

Added client command 'exit' as an alias for 'quit'.

Edits at server-actions.h#65 and server-users.h#104 are minor fixes.
Improved authentication procedure.

Client now checks server connection before asking for username and
password.

A new command is introduced: 'AUTH' is an empty request with the only
purpose of testing username-password pair. It is sent by client as soon as
these data have been typed by user. If HMAC is correct, server answers with
an empty response (only the \0x3 guard is sent), otherwise the standard
error message followed by \0x4 is sent and client exits normally.