Minor bugfix: 4 x 2 = 8, not 16

Use BBS installation check to see if we need to hook INT19 even on a PnP
BIOS.

Verify that $PnP signature is paragraph-aligned; bochs/qemu BIOS provides
a dummy $PnP signature with no valid entry point, and deliberately
unaligns the signature to indicate that it is not properly valid.

Print message if INT19 is hooked.

Attempt to use PMM even if BBS check failed.

Move the printing functions from pxeprefix.S into libprefix.S, so they
are available for debug from any prefix.

ROM initialisation vector now attempts to allocate a 2MB block using
PMM.  If successful, it copies the ROM image to this block, then
shrinks the ROM image to allow for more option ROMs.  If unsuccessful,
it leaves the ROM as-is.

ROM BEV now attempts to return to the BIOS, resorting to INT 18 only
if the BIOS stack has been corrupted.

compressor fixup section.

flow of control through this code.

We now always add PCI and PnP headers even for ISA ROMs; there's no harm
in doing so.

UNDI loader is still missing.

mechanism has changed anyway, and so must be rewritten, but at least doing
this makes the output of "make symcheck" more legible.

 callback mechanism.