debuginfod: pass a distro-summary User-Agent request header

debuginfod: pass a distro-summary User-Agent request header

It may be useful for a debuginfod server operator to know what kinds
of clients make webapi requests.  This is mainly as a
telemetry/diagnostic (though the data cannot be really trusted).  It
may also be useful to automate downloading of distro packages to a
debuginfod server in the case of an unknown hex buildid.  doc/testing
not affected as these are diagnostics.

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