Switch to allow header timestamps to be UTC

[fireflycons]

Perhaps I'm missing something, but I could not see how to get the header timestamp to be always UTC instead of system local time.

This PR adds a boolean field to Logger so that UTC may be enabled, and the logic for this in header()

Additionally fix a typo in Logger comments and disable running of syslog tests on Windows.

[phuslu]

thanks! will go through it today!

[fireflycons]

Out of interest, what reference did you use to craft the ASM to get the go-id?

[phuslu]

I added comment to goid.s, using pure asm to impalement this function can minimalized the function call overhead in go, I think.

[phuslu]

After consideration I thin it's no problem for adding the "TimeUTC" field.

1. Is there a better name for it? (If currently no, let us merge it first)
2. Maybe the code change could be simplify/tidy. (maybe I'm wrong, but it will not be a blocker)

[phuslu]

Let me merge it first, thanks.

[phuslu]

Another option is change TimeUTC bool to TimeLocation *time.Location.

If user give a non-nil value(e.g. Time.UTC), we will call Time.In for the timestamp, and we also need implement a fast path for time.UTC.

[phuslu]

@fireflycons please take a look this approach #66 if you have time, thanks.