fix #4811: rule db: test regex validity on save

fix #4811: rule db: test regex validity on save

and warn only when it's an invalid regex on execution, because users may
have previously had such rules. Otherwise, pmg-smtp-filter will restart
every time it encounters such a rule.

When testing, 'die' if the regex execution 'warns', so that users cannot
enter a semi-invalid or very wrong regex like '^*foo$'.

do so for every rule type that uses a regex to match

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
[S.I.: add short comment in test_regex sub ]
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>