Fix SDL crash:

Using werase instead of wclear, as every other part of the code does.
Also fixing clearok - there is *no* guarantee that the window has at least window::y lines.
Move motd and credits declarations outside of game::opening screen
Added refresh funcitions for motd and credits files.
All variables and functions have prefix mmenu_*.
Refresh motd/credits after lang change. Dynamically.
Fixed cataclysm-launcher to properly open symlinks.
Merge remote-tracking branches 'dda/pr/8582', 'dda/pr/8599' and 'dda/pr/8656' into dda-master

* dda/pr/8582:
  Fix SDL crash:

* dda/pr/8599:
  Refresh motd/credits after lang change. Dynamically.
  Added refresh funcitions for motd and credits files. All variables and functions have prefix mmenu_*.
  Move motd and credits declarations outside of game::opening screen

* dda/pr/8656:
  Fixed cataclysm-launcher to properly open symlinks.