Support forwarding container logs to syslog

Docker has feature to view container logs (output to stdout and stderr),
but it is not very convenient. This patch adds an option to enable
forwarding of log messages to syslog server.

To enable syslog forwarding, add "--syslog" to docker's "run" command's
arguments. It will forward messages to local (host's) syslog server. To
forward to remote syslog, use "--syslog-url=udp://server:port"
parameter.

Other options allow configuring syslog parameters:
* --syslog-facility
* --syslog-severity
* --syslog-tag
Who is "him" referring to? Do you mean something like "The `--syslog-tag MONGO` flag will add a tag, `MONGO`, to the selected log messages"?