reindex: drop notmuch_param_t, use notmuch_indexopts_t instead

reindex: drop notmuch_param_t, use notmuch_indexopts_t instead

There are at least three places in notmuch that can trigger an
indexing action:

 * notmuch new
 * notmuch insert
 * notmuch reindex

I have plans to add some indexing options (e.g. indexing the cleartext
of encrypted parts, external filters, automated property injection)
that should properly be available in all places where indexing
happens.

I also want those indexing options to be exposed by (and constrained
by) the libnotmuch C API.

This isn't yet an API break because we've never made a release with
notmuch_param_t.

These indexing options are relevant in the listed places (and in the
libnotmuch analogues), but they aren't relevant in the other kinds of
functionality that notmuch offers (e.g. dump/restore, tagging, search,
show, reply).

So i think a generic "param" object isn't well-suited for this case.
In particular:

 * a param object sounds like it could contain parameters for some
   other (non-indexing) operation.  This sounds confusing -- why would
   i pass non-indexing parameters to a function that only does
   indexing?

 * bremner suggests online a generic param object would actually be
   passed as a list of param objects, argv-style.  In this case (at
   least in the obvious argv implementation), the params might be some
   sort of generic string.  This introduces a problem where the API of
   the library doesn't grow as new options are added, which means that
   when code outside the library tries to use a feature, it first has
   to test for it, and have code to handle it not being available.
   The indexopts approach proposed here instead makes it clear at
   compile time and at dynamic link time that there is an explicit
   dependency on that feature, which allows automated tools to keep
   track of what's needed and keeps the actual code simple.

My proposal adds the notmuch_indexopts_t as an opaque struct, so that
we can extend the list of options without causing ABI breakage.

The cost of this proposal appears to be that the "boilerplate" API
increases a little bit, with a generic constructor and destructor
function for the indexopts struct.

More patches will follow that make use of this indexopts approach.