chore(logrotate): make the default value show the separator it documents

[zebby76]

Follow-up to #58, spotted while reviewing what actually reaches production.

The splitting fix accepts both forms: a value containing ; or a newline is split on it, anything
else falls back to whitespace — which keeps every value that worked before working. The shipped
default stayed on the whitespace form.

That is the wrong thing to hand someone. The default is what gets copied and extended, and copying
it to add a directive with an argument produces

"compress copytruncate missingok notifempty maxage 7"

which has no ;, takes the whitespace path, and renders 7 and maxage as two lines. Since #58
the container refuses to boot on that rather than rotating nothing in silence — so the failure is
loud — but the value it was copied from is what led there.

Change

The default is semicolon-separated. Nothing about the parsing changes:

LOGROTATE_DEFAULT_OPTIONS	result
the new default, untouched	rotates, stanza unchanged
the default copied and extended with maxage 7	rotates, maxage 7 intact
an old whitespace override	rotates, stanza unchanged

The README row said "space-separated". It now names the separator, gives an example of a directive
that takes an argument, and mentions that the rendered stanza is validated at startup.

Related, in the sibling image

base-nginx never received #58 at all — 70-logrotate.sh there still word-splits and sorts, with no
validation. Its twin is going up separately.