Add `site_log_dir()` counterpart for `user_log_dir()`

[PhrozenByte]

The user_log_dir() method currently lacks a site_log_dir() counterpart.

Some suggestions:

• Unix: /var/log/$appname?
• Android: user_log_dir() (as with any other site_*_dir() counterpart)?
• Windows: site_data_dir() with an opinionated Log subdirectory?
• Mac OS: /Library/Logs/$appname?

This is a prerequisite for #213