pid1: fully disable coredumping to $PWD

We have three states:
- ENABLE_COREDUMP and systemd-coredump is installed,
- ENABLE_COREDUMP but systemd-coredump is not installed,
- !ENABLE_COREDUMP.

In the last case we would not do any coredumping-related setup in pid1, which
means that coredumps would go to to the working directory of the process, but
actually limits are set to 0. This is inherited by children of pid1.

As discussed extensively in https://github.com/systemd/systemd/pull/26607, this
default is bad: dumps are written to arbitrary directories and not cleaned up.

Nevertheless, the kernel cannot really fix it. It doesn't know where to write,
and it doesn't know when that place would become available. It is only the
userspace that can tell this to the kernel. So the only sensible change in the
kernel would be to default to '|/bin/false', i.e. do what we do now.

In the middle case, we disabled writing of coredumps via a pattern, but raise
the RLIMIT_CORE. We need to raise the limit because we can't raise it later
after processes have been forked off. This means we behave correctly, but allow
coredumping to be enabled at a later point without a reboot.

This patch makes the last case behave like the middle case. This means that
even if systemd is compiled with systemd-coredump, it still does the usual
setup. If users want to restore the kernel default, they need to provide two
drop-in files:
  for sysctl.d, with 'kernel.core_pattern=core'
  for systemd.conf, with 'DefaultLimitCORE=0'.

The general idea is that pid1 does the safe thing. A distro may want to use
something different than the systemd-coredump machinery, and then that would
could packaged together with the drop-ins to change the configuration.

Alternative-for: #26607

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