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.
The extended-join client capability extends the JOIN message with information clients typically
query using WHO including accountname, signon TS and realname.
The code behind this capability was never implemented, and subsequent
discussions have agreed to approach the problem differently. There seems no
reason to continue advertising a capability that does nothing.
identd and SASL can cause source_p->user to be present without USER having been sent.
Without this change, that could cause a crash later on as localClient->fullcaps is not initialised.
