bus: Try to raise soft fd limit to match hard limit

Linux systems have traditionally set the soft limit to 1024 and the hard limit to 4096. Recent versions of systemd keep the soft fd limit at 1024 to avoid breaking programs that still use select(), but raise the hard limit to 512*1024, while in recent Debian versions a complicated interaction between components gives a soft limit of 1024 and a hard limit of 1024*1024. If we can, we might as well elevate our soft limit to match the hard limit, minimizing the chance that we will run out of file descriptor slots.

Unlike the previous code to raise the hard and soft limits to at least 65536, we do this even if we don't have privileges: privileges are unnecessary to raise the soft limit up to the hard limit.

If we do have privileges, we also continue to raise the hard and soft limits to at least 65536 if they weren't already that high, making it harder to carry out a denial of service attack on the system bus on systems that use the traditional limit (CVE-2014-7824).

As was previously the case on the system bus, we'll drop the limits back to our initial limits before we execute a subprocess for traditional (non-systemd) activation, if enabled.

systemd activation doesn't involve us starting subprocesses at all, so in both cases activated services will still inherit the same limits they did previously.