ports: native version of DosBox

DosBox is DOS-Emulator which is mainly used for playing old
DOS games on POSIX systems and newer Windows versions.

This port of DosBox runs natively on Genode by using its
SDL backend. It is currently only works on x86_*.

Fixes #937.
hw: remove unused function handle_invalid_syscall

ref #874
hw: remove useless function handle_invalid_excpt

ref #874
hw: header and source file for Kernel::Vm

ref #874
hw: make Kernel::mtc() global

ref #874
hw: make syscall backend private to Kernel::Thread

ref #874
hw: clean up interface of Kernel::Thread

ref #874
hw: send request size through UTCB

ref #874
hw: send reply size & receive request size by UTCB

ref #874
hw: manage UTCB through use-case specific structs

ref #874
hw: communicate message type through UTCB

ref #874
hw: distinct pagefault and IPC message type

ref #874
arndale: centralize spec of interrupt names

ref #874
pbxa9: centralize spec of interrupt names

ref #874
imx53: remove unused IPU-interrupt specs

ref #874
panda: centralize spec of interrupt names

ref #874
omap4 & gpio_drv: add config lib

ref #874
rpi: centralize spec of interrupt names

ref #874
vea9x4: centralize spec of interrupt names

ref #874
hw: handle interrupts via signals

fix #874