install() now calls relocate(), moves the protected-mode code to the new
location, and calls hide_etherboot().

from protected-mode code.

Set up %ds to point to .data16 in prot_to_real, so that code specified
via REAL_EXEC() and friends can access variables in .data16.

Move most real-mode librm variables from .text16 to .data16.

install_prealloc.  I *think* these will suffice for all the 16-bit
prefixes.

highmem data, so decompress16 will be able to unpack blocks bigger
than 64kB.

running the decompresser in 16:16 protected mode using the real-mode
stack.  However, there's an even simpler way to do it...