value-pairs: Handle SDATA enterprise IDs correctly

RFC5424 SDATA IDs are of the form "[win@18372.4 foo=bar]", which
translates to "sdata.win@18372.4.foo=bar" internally. In this case, we
should not split the "4" into a separate object, but treat it as part of
the sdata ID.

To do this, the name splitting was enhanced, and instead of splitting by
every dot, it will look for @ too, and only split a token off after
that, if a dot is followed by a non-number. A test case to verify the
behaviour is also included.

Signed-off-by: Tibor Benke <btibi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>