Remove s_assert definition from ircd_defs.h and add it to its own header.

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.