The user_state_dir() method currently lacks a site_state_dir() counterpart.

Some suggestions:

• Unix: /var/local/$appname:/var/lib/$appname multipath?
• Android: user_state_dir() (as with any other site_*_dir() counterpart)?
• Windows: site_data_dir() with an opinionated State subdirectory?
• Mac OS: site_data_dir()?

This is a prerequisite for #213