Unfuck external organs, hopefully

Created by: dylanstrategie

I can barely blame anyone for not touching this code. A single misplaced check and a lot of stuff breaks

• organ.dm and organ_external.dm now uses the helper procs in the later. Added a few helper procs along the way
• is_usable() can be run to check if a limb can be used, and is now the proc that checks if you can walk on a leg. It includes every single destructive flag except BROKEN
• is_existing() is now used to make sure there is a limb here, only includes DESTROYED and CUT_OFF
• is_peg, is_robotic() and is_organic() are all self-explanatory
• The way legs and the lack thereof work has been very slightly tweaked. Having any of either your two legs or you two foot broken and not splintered will put you at risk of falling on your ass (used to be two of the four at pure random). Likewise, having any of the four limbs missing totally will prevent you from standing up altogether. So it's wheelchair, re-attachment surgery, prosthetic or peg. Or you can just crawl around
• A huge amount of formatting, string unfucking, etc.
• Fixed a potential runtime in the gender reassignment surgery last step failure due to a cucked emote proc

Fixes #5051 (closed) along the way, hopefully permanently