Propagate LD_* variables

[haampie]

LD_* variables are unset by the kernel when executing setuid binaries, but it'd be nice to propagate them to the command we're executing after dropping privileges. The only way to do that is through a non-setuid wrapper (such as squashfs-run).

Effectively: squashfs-mount [file] [location] env "LD_XXX=$LD_XXX" [command...]