MEDIUM: log: use log_format for mode tcplog

MEDIUM: log: use log_format for mode tcplog

Merge http_sess_log() and tcp_sess_log() to sess_log() and move it to
log.c

A new field in logformat_type define if you can use a logformat
variable in TCP or HTTP mode.

doc: log-format in tcp mode

Note that due to the way log buffer allocation currently works, trying to
log an HTTP request without "option httplog" is still not possible. This
will change in the near future.