urlview: Simple xdg-open

* Invoke xdg-open and keep things simple.
shell_env: Remove keychain functionality

Disable in favor of zsh + prezto ssh functionality and death of
GPG_AGENT_INFO in GPG 2.1
shell_env: Enable multi-threaded xz/liblzma operation

* Attempt to enable multi-threaded compression (working) and
  decompression (not working?1) in xz 5.2.0.
* Hopefully this will become the default in the future.