XDG_CACHE_HOME support

[CosmicToast]

It's kind of a shame to fill up ~, especially for truly non-essential files like caches.
There's a bunch of different ways to support the XDG_CACHE_HOME standard, so even if I'm willing to draft a PR for this, I'd rather know what the project actually wants.

The typical options are:

• Simply default to the (shell-equivalent) of ${XDG_CACHE_HOME:-~/.cache}/openrdap/ instead of ~/.openrdap.
• Do the above, but only when XDG_CACHE_HOME is set.
• Do the above, but only when $XDG_CACHE_HOME/openrdap/ already exists (this is usually more useful for XDG_CONFIG_HOME).
• Provide an OPENRDAP_DIR or OPENRDAP_HOME environment variable. While it's a bit of a pain for things like login shells, at least it offers some way towards a resolution.

The interest in the case of openrdap specifically is:

• The user can now clean caches predictable by doing rm -rf $XDG_CACHE_HOME.
• Non-essential files don't take up undue amounts of attention in ~.