Fork log/syslog in order to support custom tags.

The current log/syslog package does not allow modification of the syslog
tag after a network connection is dialed. Normally this is okay, but since
logspout monitors several different processes and forwards them, we also
need to tag each process.

Additionally, tag and priority are public fields now, so if there's a need to
change the default tag and priority, it can be done.

Also, dialing every time we need to send something to syslog sucks.

Signed-off-by: Robert Xu <robxu9@gmail.com>
A client application that takes advantage of syslog would probably mutex the writer on their own in order to make changes to the defaults... or I'd probably trust them to keep it synchronized themselves. If we don't add that complexity though, I assume then we just let people redial for another default priority or tag?

I'll make it private again in the next commit.