debuginfod 2/2: server side

Add the server to the debuginfod/ subdirectory.  This is a highly
multithreaded c++11 program (still buildable on rhel7's gcc 4.8,
which is only partly c++11 compliant).  Includes an initial suite
of tests, man pages, and a sample systemd service.

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