This commit makes the ventcrawling system operate independant of
ventcrawling verbs. Every type of /mob/living now has a ventcrawling
variable, which allows them to interact with the ventcrawling system if
set to 1 or 2. The AltClickOn() defines for each ventcrawling mob have
been removed, and replaced with a single AltClick() define on ventcrawl
machinery.
1266df1f3d