Modify difficulty (mostly makes it easier)

[cooljeanius]

This comes from cooljeanius/wesnoth_mods#6 and is meant to close it. Fuller change history can be found in that repository. The map change for S20 is to turn some Orcish Castle tiles into Orcish Keep tiles, to give the player more options for where they might want to recruit from. In S05a I tried to make the different sides fight each other more. In S16 I made your allies protect villages more. In S19, Erathan kept dying on me, so I buffed his HP on Easy. In S20, for changes to the WML, I had a hard time breaking through on the single-tile wide bridges into the fortress, so originally I was going to add a way to refill the ammo on your explosive bow, but I couldn't figure out how to do that, so instead I copied over the Clockwork Dragon from Ashen Hearts, since that has a "line damage" special that lets you strike multiple tiles deep into the column of units on the bridge. It doesn't necessarily have to be the Clockwork Dragon per se; if you can think of another unit with the "line damage" special that would fit the flavor better, that could work, too. Other than that, the rest of the changes here are mostly just gold/income/turnlimit tweaks.