These functions are really specific to parsing and creating CTCP SED messages, which means they belong
in ctcp.c with the other CTCP code.
Also remove unnecessary inclusions of encrypt.h and ctcp.h.
Use size_t for passing buffer lengths, and const char * for encryption keys and other non-modified buffer
arguments.
Remove pointless helper function do_crypt().
Tokens that begin with two underscores __* or an underscore and an uppercase letter _X* are reserved,
so we should avoid those for our own include guards. The standard I'm settling on for foo.h is FOO_H_.
