debuginfod: Accelerate traversal, shutdowns, improve metrics

debuginfod: Accelerate traversal, shutdowns, improve metrics

Added new metrics for scanning that allow estimation of its reading
bandwidth.  Accelerated responsivity to SIGINT shutdown during
archive-scanning phase, which previously insisted on completely
processing the current archive.  Noted in systemd service file that in
the worst case, it might still take a long time.  Accelerated
traversals by moving regex -I/-X handling to apply to file names only
(as always documented), so directory traversal metrics are accurate
regardless of their name.

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