ConfigDirs and DataDirs return Unix paths on every platform, including Windows #437

Epic: #430 · Feature: #431

pkg/xdg/xdg.go returns the specification defaults /etc/xdg and
/usr/local/share:/usr/share. Correct on Linux and Darwin; wrong twice on Windows -- the directories do
not exist, and a leading separator with no volume is drive-relative.

The package documents the invariant these break: no anchor can resolve to a relative path.
TestNoAnchorIsEverRelative proves it for the five base accessors and does not cover these two --
the gap that let it ship.

Exposure is zero today (no callers outside the package's own tests), which is why it survived and why
it is cheap now and expensive to inherit: a system-wide install makes DataDirs() the natural place to
look for machine-wide completions and man pages.

Windows has no search-path concept to copy, so the semantics need researching against comparable tools
the way step 54's layout ruling was -- not picked for tidiness.

Charter: step 59.