Fix many package descriptions

(My OCD kicked in today...)

Remove repeated package names, capitalize first word, remove trailing
periods and move overlong descriptions to longDescription.

I also simplified some descriptions as well, when they were particularly
long or technical, often based on Arch Linux' package descriptions.

I've tried to stay away from generated expressions (and I think I
succeeded).

Some specifics worth mentioning:
 * cron, has "Vixie Cron" in its description. The "Vixie" part is not
   mentioned anywhere else. I kept it in a parenthesis at the end of the
   description.

 * ctags description started with "Exuberant Ctags ...", and the
   "exuberant" part is not mentioned elsewhere. Kept it in a parenthesis
   at the end of description.

 * nix has the description "The Nix Deployment System". Since that
   doesn't really say much what it is/does (especially after removing
   the package name!), I changed that to "Powerful package manager that
   makes package management reliable and reproducible" (borrowed from
   nixos.org).

 * Tons of "GNU Foo, Foo is a [the important bits]" descriptions
   is changed to just [the important bits]. If the package name doesn't
   contain GNU I don't think it's needed to say it in the description
   either.
pythonPackages.python_magic: 0.4.3 -> 0.4.6
remove icecat3
added xinput_calibrator v0.7.5
Merge pull request #3771 from bjornfor/fix-package-descriptions

Fix many package descriptions
Initial packaging for pnmixer
pnmixer: Add maintainer and fix name
pnmixer: Add description field
pnmixer: cleanup
remove pythonPackages.nose2Cov cc @chaoflow
ppsspp: update to 0.9.9.1
xskat: new package

Play the german card game Skat against the AI or over IRC.
Update jigdo
pythonPackages.polib: 1.0.1 -> 1.0.4
pythonPackages.supervisor: 3.0 -> 3.1.1