Xenos fixes, dev freeze fixes.

Fixes #6689, #6690, #6695
Fixes everything in #6692 other than unreproducable balance issues ('dying in one hit', movement speed). Fixes mouse ventcrawl.
Fixes human ventcrawl.
Adjusts set_species to handle verb and icon changes in the proc instead of the datumvars call. 
Removes HAS_TAIL in favour of just checking if species.tail is set.
Moves says/asks/exclaims to the language datum.
Adds checks for species-level inv slots before restricting pockets and belts based on lack of uniform/suit.
Also, this should probably be visible_message() instead.