We want to make sure everything stays below 7C00h during the
shuffle_and_boot operation.

Remove the last ad hoc protected-mode entry, in trampoline_to_pm.

Now all protected-mode entry and exit is via enter_pm/exit_pm.

Unify the protected-mode entry and exit code.  We now always provide
interrupt thunking services, even during bcopy.  That way we never
leave interrupts disabled for too long, even in extreme circumstances.

This should permit portions of the core to be written as
protected-mode code, which could be compiled with gcc if desirable.

This removes the RBFG hack from PXELINUX.  As far as I can tell, RBFG
is no longer widely used, with Etherboot-based solutions being used
instead.

Allow MENU EXIT to "exit" to an arbitrary menu.  This is really just a
variant of "MENU GOTO", but it allows displaying an exit marker.

Two bugs:

- We didn't strip ^ symbols inheriting from MENU LABEL to MENU TITLE
- We did consider_for_hotkey() with the wrong menu (the submenu
  itself, as opposed to the parent menu.)

As part of these changes, make a function available to create an
uninitialized, mutable refstring (that can be constructed and then
used as a refstring, that is.)

When we realloc() a block larger, try to protect the free block
following it from being immediately allocated by something else by
placing it at the end of the freelist instead of the beginning.

Change the realloc() implementation to allow in-place growth.  This is
an important step in handling files without knowing their sizes a
priori.

Correctly make it so MENU TITLE can be inherited upward as MENU TITLE
unless one is provided.