units: extend stop timeout for user@.service to 120s (#4426)

units: extend stop timeout for user@.service to 120s (#4426)

By default all user and all system services get stop timeouts for 90s. This is
problematic as the user manager of course is run as system service. Thus, if
the default time-out is hit for any user service, then it will also be hit for
user@.service as a whole, thus making the whole concept useless for user
services.

This patch extends the stop timeout to 120s for user@.service hence, so that
that the user service manager has ample time to process user services timing
out.

(The other option would have been to shorten the default user service timeout,
but I think a user service should get the same timeout by default as a system
service)

Fixes: #4206