WIP: flatpak-export: Try to handle absolute symlinks from /usr/
If an host system uses absolute symlinks in /usr, if we just
bind-mount the whole directory, these symlinks will be either broken or
will point to an unexpected location.
For example Gentoo used to have a symlink /usr/lib with /usr/lib64
as a target.
If we notice an absolute symlink that points to /usr/*, we can convert
it to a relative symlink to preserve its original target even inside the
container.
Should fix #43 (moved)
This is supposed to be sent upstream, but I preferred a quick review before sending this patch. Just to be sure that the approach that I used is the correct one.
AFAIK there shouldn't be files in /usr/, just directories and symlinks. If that's not the case, we should bind them too.