Introduce new function IRC_WriteErrClient()

This function is used to send "error messages", including numerics,
back to clients and to automatically enforce a 2 second penalty. With
this patch, all error results enforces a delay for the client.

All callers of IRC_WriteStrClient(ERR_xxx) have been converted.

Please note that this patch prolongs the time "make check" needs
to complete its tests, because of lots of new enforced penalties ...
IRC_SERVER(): Code cleanup