systemd.bbclass: support user units in postinst and prerm hooks

systemd.bbclass: support user units in postinst and prerm hooks

Handle user units in a manner similar to system units where possible.
Not everything is supported by systemd, but systemd limitations only
affect runtime package management - during update user services are not
reloaded/restart and each user must re-login or manually restart
services.

Signed-off-by: Artur Kowalski <arturkow2000@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>