logger: bugfix: tcp syslog framing is broken, -T unusable

logger: bugfix: tcp syslog framing is broken, -T unusable

Logger can send via plain tcp syslog if -n -T options are given.
However, the framing is broken so that a syslog receiver can not
know where the first message ends and the next one starts. It
actually looks like no framing at all is used. Plain TCP syslog
framing is described in RFC6587.

This patch adds RFC6587 octet-stuffed framing to TCP syslog. For
local logging, this is always fine, for remote logging this is
NOT recommended by the IETF (the RFC is historic). However, a
full blown RFC5425 TLS sender seems to be out of scope for a tool
like logger IMO.

This patch also refactors the way output is written, seperating
the message format generators from the output writer.