sepolicy: Add better private API for setfscreatecon

sepolicy: Add better private API for setfscreatecon

Use `g_auto()` more sanely with a struct implmenting the "is initialized"
pattern.  This is way less ugly for callers, and fixes bugs like
us calling `setfscreatecon()` even if an error occurred beforehand.

Also fold in the logic for "NULL or not loaded" sepolicy into the setup rather
than requiring callers to inline it.

Prep for more users of this function.

Closes: #746
Approved by: jlebon