nixpkgs: Bump screen to 4.2.1

Builds fine locally.

Dropped both patches, because

1) Arch Linux (the source of these according to the first line in both) dropped them as well

2) The caption colors patch was applied upstream (see [1])

3) The terminal name length was solved with a single constant upstream (instead of patching two places)

1: http://git.savannah.gnu.org/cgit/screen.git/tree/src/screen.c#n2238