muniversal PortGroups don't restore file permissions

[RJVB (René Bertin)]

I just realised that using a muniversal PG on port that generates a setuid executable breaks that aspect of the port's functionality.

The ports in question could of course fix this in the post-destroot step but IMHO it would be cleaner if the permissions were cached before and restored after the lipo operation.

I'd read the permissions from the file corresponding to the current build platform but a more complete/pedantic implementation would check if all files to be lipo'd had the same permissions.