Extra boundary checking on g_vertical_offset and new flag -keymapfile

[DirtBagXon]

Added extra boundary checking on g_vertical_offset negative values causing segfault. And restrict input range of -vertical_stretch from 0 to 25 to this cause.

Pushing back as contains extra checks on earlier pull request,

HOWEVER, also adds a new flag (feature) for alternate keymap files. Allows specifying UP/DOWN swap keymap files for flight based games, as an example

Not certain you want to accept feature additions, or indeed if this should include a 1.0.13.1 version change.

Can be addition fields in run.sh

    astron|blazer|cobra|galaxy|interstellar|mach3|uvt)
        VLDP_DIR="vldp"
        DAPINPUT="-keymapfile flight.ini"
        ;;

./$DAPHNE_BIN $1 vldp \
$FASTBOOT \
$FULLSCREEN \
$DAPINPUT \
. . .

[DavidGriffith]

Since it doesn't seem that Matt is keeping Daphne up to date, I figure it would be okay to add new features here.