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.