Many of the external links found in the notmuch source can be resolved
using https instead of http. This changeset addresses as many as i
could find, without touching the e-mail corpus or expected outputs
found in tests.
Since xapian provides the ability to restrict the iterator to a given
prefix, we expose this ability to the user. Otherwise we mimic the other
iterator interfances in notmuch (e.g. tags.c).
This is a thin wrapper around the Xapian metadata API. The job of this
layer is to keep the config key value pairs from colliding with other
metadata by transparently prefixing the keys, along with the usual glue
to provide a C interface.
The split of _get_config into two functions is to allow returning of the
return value with different memory ownership semantics.
