Added cf_section_parse_pass2() for pass2 checks of config items

and PW_TYPE_XLAT, too.

Modules can now tell the server core that config items will
be dynamically expanded.  For now, this means that the server
can parse them in pass2, and complain at compile time about
syntax errors.

We will later expand this to allowing CONF_PAIRs to have compiled
xlat's, which should be faster at run time
Call cf_section_parse_pass2() after loading all of the modules
Add PW_TYPE_XLAT to most modules

We still need to convert the SQL queries, as they are buried
inside of randomly named sections