loader: add HTTP support using UEFI

Add support for an HTTP "network filesystem" using the UEFI's HTTP
stack.

This also supports HTTPS, but TianoCore EDK2 implementations currently
crash while fetching loader files.
Only IPv4 is supported at the moment. IPv6 support is planned for a
follow-up changeset.

Note that we include some headers from the TianoCore EDK II project in
stand/efi/include/Protocol verbatim, including links to the license instead
of including the full text because that's their preferred way of
communicating it, despite not being normal FreeBSD project practice.

Submitted by: scottph
Reviewed by: imp, bcran
Differential Revision: https://reviews.freebsd.org/D20643