Still parse messages that don't contain an RFC 3164 timestamp.

The changes made in r326573 required that messages always start with an
RFC 3164 timestamp. It looks like certain devices, but also certain
logging libraries (Python 3's "logging" package) simply don't generate
RFC 3164 formatted messages containing a timestamp.

Make timestamps optional again. When the timestamp is missing, also
assume that the message contains no hostname. The first word of the
message likely already belongs to the message payload.

PR: 229236
Reported by: Michael Grimm & Marek Zarychta
Reviewed by: glebius (cursory)
MFC after: 1 week