Check if symlink target for /etc/localtime exists in the runtime
Check if symlink target for /etc/localtime exists in the runtime

On unmonitored systems (if the session helper is not available), we
currently decide whether to create a symlink or a read-only bind mount
for /etc/localtime based on whether that's a symlink in the host or
not, but we don't check whether that symlink would be reachable in
the sandboxed environment, which might lead to bad situations.

This patch adds some extra checks relying on GFile's API to check
whether that symlink would be reachable before making the final
decision: if it's not, then do a bind mount despite of the file
being a symlink in the host.

https://github.com/flatpak/flatpak/issues/409

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak