common/options: new options infrastructure

common/options: new options infrastructure

Define schema for config options.  Helper to generate a header fragment
to declare the types.

Unlike the old config_opts.h approach, we will not intialize values in
the header.  This avoids a recompile if there is a change and also allows
us to specify different defaults and do parsing and validation at runtime.
Instead, we'll intialize values in the constructure of the containing
class.

Signed-off-by: Sage Weil <sage@redhat.com>