lib: Drop GFile variant mapping API for fd-relative

lib: Drop GFile variant mapping API for fd-relative

In addition to generic fd relative porting,
this is a necessary preparatory step for libglnx porting, because
when I tried to use `g_mapped_file_new` I hit an issue with
it using a different error domain from GIO.

Thankfully libglnx consistently uses the GIO error domain, and here
we're now using it for the `open()` call.

Closes: #317
Approved by: jlebon