loader.efi: Recognize new memdisk=<url> and memcd=<url> options

Support ipxe downloading of a memory disk (either presented to the OS as
a harddisk or a cd). This requires an ipxe server since it uses the ipxe
download protocol to grab the disk. If there is a disk, we add it to the
environment as a disk, and then the rest of the bootloader just sees it
and boots from it.

I've cribbed code from https://github.com/russor/memdisk_uefi and
adapted it to work in the context of the FreeBSD bootloader.

The ipxe_download.h file was created from the documentation of the
interface.

So a .ipxe file with the line
chain http://10.2.0.1/loader.efi memdisk=${cwduri}FreeBSD-15.1-RELEASE-amd64-bootonly.iso
would use the FreeBSD boot loader to boot the FreeBSD 15.1 release.

md(9) has a bug at the moment that prevents it from probing the
partitions on it.

Also, we'll automatically decompress gzip, bzip2 and zstd files
automatically.

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D57677

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