PR29472: debuginfod: add metadata query webapi, C api, client

PR29472: debuginfod: add metadata query webapi, C api, client

This patch extends the debuginfod API with a "metadata query"
operation.  It allows clients to request an enumeration of file names
known to debuginfod servers, returning a JSON response including the
matching buildids.  This lets clients later download debuginfo for a
range of versions of the same named binaries, in case they need to to
prospective work (like systemtap-based live-patching).  It also lets
server operators implement prefetch triggering operations for popular
but slow debuginfo slivers like kernel vdso.debug files on fedora.

Implementation requires a modern enough json-c library, namely 0.11,
which dates from 2014.  Without that, debuginfod client/server bits
will refuse to build.

% debuginfod-find metadata file /bin/ls
% debuginfod-find metadata glob "/usr/local/bin/c*"

Refactored several functions in debuginfod-client.c, because the
metadata search logic is different for multiple servers (merge all
responses instead of first responder wins).

Documentation and testing are included.

Signed-off-by: Ryan Goldberg <rgoldber@redhat.com>
Signed-off-by: Frank Ch. Eigler <fche@redhat.com>