[SRU] Add -E support to lxqt-sudo

[ Impact ]

LXQt Sudo is a tool used by the Lubuntu desktop to authorize applications requiring escalated privileges. While presenting a graphical prompt (making it useful for graphical applications), it uses sudo/su/etc. on the backend to actually honor that request.

It recently came to my attention during the review process for bug 2038958 that lxqt-sudo does not pass enough environment variables. Rather than taking the approach of replacing that command with pksudo (or even a direct sudo), this adds a feature to LXQt Sudo allowing those environment variables to pass through. As a simple, opt-in command line option, this will not affect existing users who don't know about it.

I've submitted the patch upstream for review. As is usual when I submit patches, it'll probably be rejected, and we'll keep it as a vendor patch anyway. https://github.com/lxqt/lxqt-sudo/pull/204

[ Test Plan ]

 * Launch QTerminal.
 * Run `lxqt-sudo qterminal` (or some other graphical app)
   - No XDG-specific environment variables will be present when running `env`
 * Run `lxqt-sudo -E qterminal`
   - When running `env`, all environment variables present for the user should now be present on that child process.

Both of those commands should run without issue.

[ Where problems could occur ]

 * If lubuntu-update-notifier is not released after (or with) lxqt-sudo, it will fail to run. Explicit dependencies are set, so apt should prevent this.
 * If someone calls lxqt-sudo ARGS COMMAND when they really meant lxqt-sudo COMMAND ARGS, they could trigger this on accident. This scenario is unlikely.
 * As noted in some of the code comments on the file already, some applications will break when ran with this. That being said, they're accepting the same risk they would otherwise with `sudo -E`, so this is more of a calibration in user expectations than it is a bug.
 * If Qt behavior changes drastically, or its underpinnings, the environment variables have the potential not to be read.

[ Other Info ]

In case you're wondering why -E isn't passed through *directly* to sudo (or similar), it's because the environment variables are being read into an array anyway, for string escaping and such. (I'd just like to keep the impact of this change low.)