debuginfod: print U-A: and X-F-F: request headers

debuginfod: print U-A: and X-F-F: request headers

For an incoming webapi request, print two headers that should assist
in the administration of a debuginfod service.  At fweimer's
suggestion, added a bit of filtering so the text is more reliably
parseable.

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