[z1871] Implement Condition CONDITION_SKILL_BELOW to be able to check if a player has a skill lower than a given value

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

(based on commit 64d9af6)

Signed-off-by: stfx <stfx@hotmail.de>

(cleaned version of 0816513)

(based on commit [s1437] - d600bcc)