nspawn: deprecate --share-system support

nspawn: deprecate --share-system support

This removes the --share-system switch: from the documentation, the --help text
as well as the command line parsing. It's an ugly option, given that it kinda
contradicts the whole concept of PID namespaces that nspawn implements. Since
it's barely ever used, let's just deprecate it and remove it from the options.

It might be useful as a debugging option, hence the functionality is kept
around for now, exposed via an undocumented $SYSTEMD_NSPAWN_SHARE_SYSTEM
environment variable.