Fixed "opam init" for fish and csh

* removed sourcing of complete.sh and switch_eval.sh in init.{csh,fish}
  for fish and csh, since it doesn't work. Fixes #1614
* output correct string for fish "eval (opam config env)"
* fix setting environment variables for fish
* fix output to config.fish, and actually create init.fish.
  This fixes #952

also, I created a pull request for an initial attempt at tab completions for fish at fish-shell/fish-shell#1615
I usually value patch conciseness over code alignment (no need to rewrite your PR for that though)