that could be defined in `.zimrc`, when doing the zcompile in
the completion module and when cleaning the cache.
A custom `${zcompdump_file}` would not be zcompiled or cleaned from the
cache without this correction.
Also, use zrecompile instead of the custom logic to call zcompile.
And fix init.zhs to export ZIM_HOME. The zmanage script invokes a new
shell for the sub commands, and those would fail as there would be no
value for ZIM_HOME.
Closes#203
