document-portal: Make fd validation stricter wrt /proc/pid/fd symlink
document-portal: Make fd validation stricter wrt /proc/pid/fd symlink

The symlink check is quite safe in the sense that it doesn't call out
to anything weird, so it is quite reliable, and by ensuring it is
in a non-deleted, absolute pathname form we can get rid of many weird
file descriptors without any unsafe i/o.

flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal