Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.
Change all pwd calls for $PWD variable

This avoids spawning additional processes as $PWD **always** contains
the current working directory.
Change all hostname calls for $HOST or $SHORT_HOST
Merge pull request #3223 from mcornella/use-special-vars

Use special variables where we can
Brings all directory stuff to directories.zsh

- Cleans out aliases.zsh.
- Removes unneeded cd function.
Pushes autoenv code to its plugin.

Sources the activate.sh file or informs the user, that its not
available.
Overriding the cd function, as it was formerly done, is not needed
anymore, activate.sh does this by itself.
Refactors history alias and moves it.

More compact case statement instead of the ugly if elif... else
statement. Moves it to history.zsh
Moves remaining aliases to misc and deletes file.

There were only three aliases left in aliases.zsh
File deleted!
Moves a bindkey call from misc to key-bindings.
Deletes dot aliases from common-aliases.

They are implemented in lib anyway.
Fix a typo