When sending an error use the IRC_WriteErrClient() variant

This ensures that all errors have a 2 second penalty.
Introduce a macro to define the command list

This will pave the way to other changes and simplifies the entries somewhat.
Move IRC_SetPenalty() to Handle_Request() when possible

This centralizes the penalty handling. It also exposes some commands that
lack it for our attention.
Rework the penalty handling slightly

Increase the penalty for a command before checking its arguments.  This
makes the handling more consistent and allow us to move more penalties to
Handle_Request().
Add comments around some IRC_SetPenalty() calls
Remove unused vsnprintf.c test function
vsnprintf.c: make code compatible with ansi2knr tool
conf-ssl.h: make code compatible with pre-ANSI C compilers
Only use unsetenv() when it is available

AIX 4.3 dosn't support it, for example.
configure: clean up function definitions
doc/Platforms.txt: add OpenBSD 5.4 on HPPA
doc/Platforms.txt: add DragonflyBSD 3.4 on x86_64

Thanks to Götz Hoffart <goetz@hoffart.de>!