Fix missing mapgen keys

[jbytheway]

Summary

SUMMARY: Bugfixes "Fix assorted minor mapgen errors"

Purpose of change

There was a lot of mapgen which used characters in the "rows" layout with no definition in the "terrain", "furniture", etc. I have a branch which makes such undefined characters an error, but before I can PR that I need to fix all the existing errors of that sort.

This PR is the first step: fixing all the examples in the core mapgen (i.e. non-mods).

Describe the solution

For each undefined character, try to figure out how to fix it. Fixes roughly fall into a few categories:

• The character was a typo for some other character, change it.
• The definition for the character was missing; add it (often based on similar mapgen elsewhere from the same designer).
• The character appeared to be just a placeholder or obsolete, or I just couldn't figure out what it was supposed to be for, in which case just replace it with either . or , the characters I've reserved for background fill_ter and which will not cause an error if undefined.
• The character is a visual indicator to help understand the map layout. In this case just define it to be fill_ter.

Describe alternatives you've considered

All of these changes are somewhat subjective judgement calls. In some cases I was able to contact the original author for clarification, but in many cases I have just made my best guess. There are certainly other alternatives.

Testing

For a couple of the maps where I was less certain about the changes I spawned them in-game, but the vast majority of these changes are untested.

Additional context

I split the changes into commits based on who I believe to be the original author / maintainer of the changed maps. Hopefully this will allow them to review more easily (Edit: I've rebased, so these commit ids are no longer the ones on this branch, but hopefully they should still exist and be essentially correct):
3372695 @Night-Pryanik
48fbdd2 @nexusmrsep
48ada88 @mlangsdorf
366db53 @LovamKicsiGazsii
b39f862 @John-Candlebury
dcf4d47 @I-am-Erk
07f7a18 @curstwist
7a9d097 @CountAlex
582bdd8 @chaisawlajatang
391f476 uncertain/mixed ownership

[curstwist]

all your corrections for me look good, thanks so much!

[I-am-Erk]

Good on my end, I also checked the unknown maps