Stop dithering over GPrivate

Stop dithering over GPrivate

Take out the half-private g_private_init() stuff and replace it with a
G_PRIVATE_INIT macro that allows specifying a GDestroyNotify.

Expose the GPrivate structure in a public header.

Add a g_private_replace() to (sort of) match the functionality of
g_static_mutex_set().

Improve the documentation.

Deprecate g_private_new().