systemctl: support --global and --root in edit and cat

systemctl: support --global and --root in edit and cat

Make bus acquisition conditional in verb_edit() and verb_cat(), following
the same pattern used in verb_enable(). When install_client_side() returns
non-zero (indicating --global, --root, offline, or similar scenarios), skip
acquiring a D-Bus connection and perform all operations client-side.

Changes:
- Only acquire bus when install_client_side() returns NO
- Use mangle_names() instead of expand_unit_names() in client-side mode
- Pass force_client_side flag based on bus availability
- Skip bus-dependent operations (need_daemon_reload, etc.) when bus is NULL

This allows 'systemctl edit --global' and 'systemctl cat --global' to work
correctly, fixing the regression introduced by commit d77d42ed3a.

Test cases added to verify:
- Creating and editing global user units with --runtime
- Reading global units with cat --global
- Proper detection and rejection of masked units in client-side mode
- Tests use /run/ instead of /etc/ for safer temporary testing

Fixes https://github.com/systemd/systemd/issues/31272