logger: support sub-trees in the ID for RFC5424

logger: support sub-trees in the ID for RFC5424

The current code supports <name>@<digit> only, but we also need
<name>@<digit>.<digit>[. ...]

RFC5424: 7.2.2 enterpriseId:

 In general, only the IANA-assigned private enterprise number is
 needed (a single number).  An enterprise might decide to use
 sub-identifiers below its private enterprise number.  If sub-
 identifiers are used, they MUST be separated by periods and be
 represented as decimal numbers.  An example for that would be
 "32473.1.2".

Addresses: https://github.com/karelzak/util-linux/issues/406
Signed-off-by: Karel Zak <kzak@redhat.com>