trivial-httpd: Add --force-range-requests option to force range requests

This will be used to test resuming interrupted downloads for
ostree-pull.

With this option, if a whole file is asked for, only half of the file
is given.  Then the client should retry with a range request, and
we'll give them the other half.

https://bugzilla.gnome.org/show_bug.cgi?id=705925