config: Abolish use of ConfigParser's DEFAULT feature

ConfigParser provides no way to find out which sections a key is
_really_ in: in all of its views, keys in DEFAULT appear aliased in
all sections.

So instead we use a COMMON section, and implement the defaulting
ourselves.  This involves:

* Changing the cfg.get[int] calls to use cfg_search (these are
  the things which implictly use DEFAULT and now need to be
  explicit).
* Changing all our explicit references to and definitions of
  DEFAULT options to COMMON.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>