s_assert requires some higher-level functionality that shouldn't be
present in ircd_defs.h. ircd_defs.h is used by ssld, which has no notion
of logging or sending IRC messages. Additionally, some of the headers
s_assert depends on result in conflicting definitions in ssld.c.
This change also fixes the compile when using --enable-assert=soft.
This has a separate enabling option channel::channel_target_change.
It applies to PRIVMSG, NOTICE and TOPIC by unvoiced unopped non-opers.
The same slots are used for channels and users.
