Running external programs on certain events.

Viewer logged in
The program should return a non-empty line on standard output to allow viewer connection or empty line to immediatly disconnect viewer.

Viewer gone
Program will be executed when viewer ends the session.

Ctrl-Alt-Del handler
Program will be executed when viewer sending combination of keys Ctrl-Alt-Del.

The tag %character can be used in commands to run programs. This tag will be replaced at run-time by a string representation of the option value:
 %h  - client host address,
 %r  - 1 for reverse connection, otherwise 0,
 %v  - 1 for view-only connection, otherwise 0,
 %i  - string obtained from "Viewer logged in" program,
 %o  - 1 when other logged-in viewers present, otherwise 0,
 %%  - '%' character.
