lib: Add `_ALLOW_NOENT` flag to internal variant mapping API

lib: Add `_ALLOW_NOENT` flag to internal variant mapping API

We have a lot of "allow_noent" type wrapper functions since
a common pattern is to allow files to not exist, but still
throw cleanly on other issues.

This is another instance of that, and cleans up duplicated error
handling code.

Part of this is prep for moving away from `GFile` consumers.

Closes: #319
Approved by: jlebon