dhclient: support option 114, default-url ascii

This will enable further automation of HTTP UEFI boot loader support by
providing a specific option for providing the boot URL to FreeBSD.

Documented in:

https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml
https://kb.isc.org/docs/isc-dhcp-44-manual-pages-dhcp-options
https://tools.ietf.org/html/rfc3679

Approved by: emaste
MFC after: 2 weeks
Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D22475