discover: Support DHCP "pathprefix" configuration option

This change implements support for the DHCP "pathprefix" option. We use
the following logic:

 - If pathprefix is present and a full URL, we base the config file
   location on pathprefix + conffile

 - If pathprefix is present but not a full URL, we use it as the path
   component of the URL, and pick up the host from other parameters in
   the DHCP response

 - If no pathprefix is present, we determine the configuration prefix
   from the DHCP bootfile parameter.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>