conf parsing: Fix memory leaks and clean up code a bit.

Charybdis currently leaks about 45-50k per configuration parse,
including every rehash. This change plugs these leaks by properly
iterating through all conf_parm_t structures to seek all strings that
should be freed and also by freeing the conf_parm_t structures
themselves.

These leaks have been present since the original rewrite of the
configuration parsing system in ircd-ratbox r11953.

Additionally, this change also cleans up and documents the parsing code
a bit.