Logging: Introduced struct log_msg.

Logging: Introduced struct log_msg.

The logging handlers and filters are often interested in various properties
of the message in their raw form. I have modified the hooks to accept
a single pointer to `struct log_msg', which contains all needed information.

An immediate consequence is that we no longer need the calls to time() in
log-file, so a message is always timestamped at a single place when entering
the log machinery.