Consider the `zcompdump_file` variable, that could be defined in
`.zimrc`, when cleaning the cache.
A custom zcompdump_file would not be cleaned from the cache without this
correction.
Also remove *.zwc.old files from cache, as zrecompile also generates
these.
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
