Bug#823530: In favor of keeping the limit

[Maik Zumstrull]

In my opinion, the bugs here are in different packages:

- Packages that provide a way for users to log in to the system, but
don't create a user slice
- Packages that provide services that operate using an unreasonable
number of processes or threads, and can't be bothered to declare as
much in the unit file

The 512 limit is arbitrary, but a plausible point at which to say, if
this isn't an interactive session and the service has not declared
special needs, I should assume it's malfunctioning / fork bombing and
shut it down.

For comparison, RabbitMQ installations also regularly run into the
"1024 open file descriptors" limit, but instead of abolishing that
limit globally, it's been documented that the limit may need to be
raised specifically for RabbitMQ for high-load installations.

Regarding the user slice thing, maybe systemd should consider
depending on libpam-systemd. It's currently quite easy to not install
on custom installations, and not having user slices should break a
number of things. (It's pulled in by ubuntu-standard on Ubuntu, but
people like to remove that because it pulls in things of questionable
usefulness.)