logger: Add support to logger for RFC6587 octet counting

logger: Add support to logger for RFC6587 octet counting

This patch adds support to logger for RFC6587 octet counting.
RFC6587 provides support for two sorts of framing:

1. Octet counting (at RFC6587 s3.4.1)

   In essence each frame is preceded by a decimal length and a
   space.

2. Non-transparent framing (at RFC6587 s3.4.2), also called
   'octet stuffing'

   In essence each frame is terminated by a `\n`

Prior to this patch, logger used option 2 (non-transparent framing)
on TCP, and used no framing on UDP. After this patch, the default
behaviour is unchanged, but if the '--octet-count' option is supplied,
option 1 is used for both TCP and UDP. Arguably octet count framing
makes little sense on UDP, but some servers provide it and this
allows testing of those servers.

Signed-off-by: Alex Bligh <alex@alex.org.uk>