Generalize ipappend handling as "sysappend", and move to PM code

Generalize ipappend handling as "sysappend", and move to PM code

Generalize the ipappend handling to cover all the derivatives, and
rename it "sysappend" ("ipappend" is a valid alias for all
derivatives.)  Move all the string handling to protected mode.
Currently only pxelinux exports strings, but the plan is to change
that in the future.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>