core: add [Enable|Disable]UnitFilesWithFlags DBUS methods

core: add [Enable|Disable]UnitFilesWithFlags DBUS methods

The new methods work as the unflavoured ones, but takes flags as a
single uint64_t DBUS parameters instead of different booleans, so
that it can be extended without breaking backward compatibility.
Add new flag to allow adding/removing symlinks in
[/etc|/run]/systemd/system.attached so that portable services
configuration files can be self-contained in those directories, without
affecting the system services directories.
Use the new methods and flags from portablectl --enable.

Useful in case /etc is read-only, with only the portable services
directories being mounted read-write.