Add a -E option, preserving all environment variables

[tsimonq2]

This functions almost identically to sudo -E. I wrote this patch in an attempt to get the Lubuntu release out, but after some testing it does work as intended.

[palinek]

I disagree with this PR.

1. You are changing the default behavior in backward incompatible manner here.
2. The switch to (not) preserving environment variables can't be supported with all backends, so we should not expose it.