Merge duplicate code.

Fixes building with gcc-4.8
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Migrate the remaining callers to FatalError().

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.