Add metapackage live-task-standard

Please consider this merge, to create a metapackage named live-task-standard. The motivation for the metapackage is to use it to create a 'standard' live image; see bug 876428.

This metapackage depends on packages with standard priority. The dependency list was initially created by mimicking tasksel's "standard task" package-selection logic, which is (roughly) this:

    grep-aptavail -s Package \( -F Priority standard \) --and --not \( -F Section '^lib|\/' \)

A few dependencies were manually pruned from the list, at the prompting of lintian.

The metapackage description was copied/adapted from tasksel's "standard task" description.

live-build does not need such a task to build an ISO with a standard system on it (cf https://live-team.pages.debian.net/live-manual/html/live-manual/customizing-package-installation.en.html#416). live-wrapper is currently unmaintained (got removed from Debian Buster due its reliance on vmdebootstrap!). So I'm not sure if it's really useful.

Thanks, @hertzog. According to Steve McIntyre on the live-build list, live-wrapper is currently still being used to create the official live images.

Steve also said that he does not have time to spend on modifying the live-ISO-creation workflow himself. So, until additional DD-level help arrives, perhaps this proposed "live-task-standard" metapackage (or something similar to it) could serve in the meantime.

That's my thought too, yes...

merged

mentioned in commit 9b0afb3b