core: allow setting WorkingDirectory= to the special value ~

      If set to ~ the working directory is set to the home directory of the
user configured in User=.

This change also exposes the existing switch for the working directory
that allowed making missing working directories non-fatal.

This also changes "machinectl shell" to make use of this to ensure that
the invoked shell is by default in the user's home directory.

Fixes #1268.