main: uid_to_name() might fail due to OOM, protect against that
coredump: add simple coredump vacuuming

When disk space taken up by coredumps grows beyond a configured limit
start removing the oldest coredump of the user with the most coredumps,
until we get below the limit again.
update TODO
coredump: fix how the compression level is verified
coredump: don't be annoyed if another coredump hook removes our coredump while we work on it
journald: invoking fstatvfs() is now redundant in the vacuuming code
coredump: don't expose the compression level as configuration option

This sounds overly low-level and implementation-detaily. Let's just
use the default level XZ suggests. This gives us more room to possibly
swap out the compression algorithm used, as the compression level range
will not leak into user configuration.
coredump: replace Compression= setting by simpler Compress= boolean setting

Let's move things closer to journald's configuration settings, which
knows Compress= already, as a boolean. This makes things more uniform,
but also gives us more freedom to possibly swap out the used compression
algorithm one day.
coredump: simplify compression logic a bit

This also make sure we remove the original coredump temporary file if we
successfully managed to compress the coredump.
update TODO