msgbuf: Allow microsecond granularity timestamps

Today, kern.msgbuf_show_timestamp=1 will give 1 second granularity
timestamps on dmesg lines. When kern.msgbuf_show_timestamp=2, we'll
produce microsecond level graunlarity.
For example:
old (== 1):
[13] Dual Console: Video Primary, Serial Secondary
[14] lo0: link state changed to UP
[15] bxe0: NIC Link is Up, 10000 Mbps full duplex, Flow control: ON - receive & transmit
[15] bxe0: link state changed to UP
new (== 2):
[13.807015] Dual Console: Video Primary, Serial Secondary
[14.544150] lo0: link state changed to UP
[15.272044] bxe0: NIC Link is Up, 10000 Mbps full duplex, Flow control: ON - receive & transmit
[15.272052] bxe0: link state changed to UP

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