Partly revert c0a2a4d again

As it turns out, zsh has defaults on HISTSIZE and SAVEHIST. I have lost track
off zsh defaults a long time ago. This is a reason not to do this at all.
HISTSIZE's default is 30, SAVEHIST's default is 0. And the latter is a big
problem: No history will be saved anymore.

If someone wants different values for these two parameters, they can set them
in .zshrc.local. I am NOT working around this by comparing the actual value to
zsh's defaults.

The combined diff of c0a2a4d and this commit looks like this:

[snip]
--- a/etc/zsh/zshrc
+++ b/etc/zsh/zshrc
@@ -1582,7 +1582,7 @@ function command_not_found_handler () {
 # history

 #v#
-HISTFILE=${ZDOTDIR:-${HOME}}/.zsh_history
+HISTFILE=${HISTFILE:-${ZDOTDIR:-${HOME}}/.zsh_history}
 isgrmlcd && HISTSIZE=500  || HISTSIZE=5000
 isgrmlcd && SAVEHIST=1000 || SAVEHIST=10000 # useful for setopt append_history
[snap]

Closes: Github bugreport #64