merge: overhaul of the config system

 * config: allow deeper inheritance
 * tests: minor fixes
 * tidy type definitions
 * move private flags
 * move sensitive flag to type
 * tidy init.h config definitions
 * move redraw flags
 * rename the flags field to data (and move it)
 * drop dt_path
 * drop dt_command
 * drop mutt dump
 * add enum type
 * add slist type
 * mbox_type -> enum
 * drop magic type
 * hidden_tags -> slist
 * mailcap_path -> slist
 * imap_authenticators -> slist
 * smtp_authenticators -> slist
 * pop_authenticators -> slist
 * preferred_languages -> slist
 * fix makedoc