zshrc: Add more robustness to completion caching setup [Closes: issue1933]

This is based on a suggestion by Thilo Six <tech@xk2c.de>.

The setup now uses $GRML_COMP_CACHING to decide on whether to configure
caching at all and $GRML_COMP_CACHE_DIR to tell where to put the caching
data.

It also makes sure the caching directory actually exists before using it.

See the included documentation for details.

Signed-off-by: Frank Terbeck <ft@grml.org>