debuginfod: Use MHD_USE_ITC in MHD_start_daemon flags

debuginfod: Use MHD_USE_ITC in MHD_start_daemon flags

This prevents the "Server reached connection limit. Closing inbound
connection." issue we have been seeing in the
run-debuginfod-webapi-concurrency.sh testcase. From the manual:

    If the connection limit is reached, MHD’s behavior depends a bit
    on other options. If MHD_USE_ITC was given, MHD will stop
    accepting connections on the listen socket. This will cause the
    operating system to queue connections (up to the listen() limit)
    above the connection limit. Those connections will be held until
    MHD is done processing at least one of the active connections. If
    MHD_USE_ITC is not set, then MHD will continue to accept() and
    immediately close() these connections.

https://sourceware.org/bugzilla/show_bug.cgi?id=28708

Signed-off-by: Mark Wielaard <mark@klomp.org>