MEDIUM: log: add a new log format flag "E"

The +E mode escapes characters '"', '\' and ']' with '\' as prefix. It
mostly makes sense to use it in the RFC5424 structured-data log formats.

Example:

log-format-sd %{+Q,+E}o\ [exampleSDID@1234\ header=%[capture.req.hdr(0)]]
5 files changed