syslog-format: accept RFC5424 NILVALUE in timestamps

RFC5424 states:

In section 6:
TIMESTAMP       = NILVALUE / FULL-DATE "T" FULL-TIME

In section 6.2.3
   A syslog application MUST use the NILVALUE as TIMESTAMP if the syslog
   application is incapable of obtaining system time.

This patch changes syslog-ng to conform to this requirement as it reported
a parse error instead.

Reported-By: Vincent Brillault <spam@lerya.net>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>