PR31862: debuginfod: client to cache x-debuginfod-* headers

PR31862: debuginfod: client to cache x-debuginfod-* headers

This feature allows the extra http headers from debuginfod to be saved
into the client cache, and also thus replayed to clients.  This way
they can perform IMA verification again, if they like, or a federating
caching intermediate debuginfod server can replay the headers it
received previously from upstream to future downstream.  The headers
are placed adjacent to the payload files .cache/debuginfod/BUILDID/PAYLOAD
as .cache/debuginfod/BUILDID/hdr-PAYLOAD.  They are aged the same
atime-based way.

Testing via an extension of a previous small test, and hand-running both
client & server code under valgrind.  No memcheck errors reported.

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