PR28661: debuginfo connection thread pool support

PR28661: debuginfo connection thread pool support

Add an option -C, which activates libmicrohttpd's thread-pool mode for
handling incoming http connections.  Add libmicrohttpd error-logging
callback function so as to receive indication of its internal errors,
and relay counts to our metrics.  Some of these internal errors tipped
us off to a microhttpd bug that thread pooling works around.  Document
in debuginfod.8 page.  Hand-tested against "ulimit -u NNN" shells, and
with a less strenuous new test case.

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