[s1944] Add support for XP-Gain disabling

This will maintain compatibility with master, nothing more

Add CONDITION_XP_USER to be able to check if a player has XP-Gain enabled or disabled
Add SCRIPT_COMMAND_XP_USER to enable/disable XP-Gain for a player

Signed-off-by: Schmoozerd <schmoozerd@scriptdev2.com>

(based on commit [12420] - 871161f)

Signed-off-by: Xfurry <xfurry@scriptdev2.com>