zsleep() and serializer msgport backend

["Sepherosa Ziehau" <sepherosa@xxxxxxxxx>]

Hi,

http://leaf.dragonflybsd.org/~sephe/msgport_slize.diff

The above patch create:
1) zsleep(), similar to msleep(), expect serializer is involved
2) serializer msgport backend, similar to spin msgport backend, except
that serializer is involved and caller is assumed to hold serializer

Please review it.

I will need these to make Intel wireless devices work properly: use
one thread to pipeline all firmware operations.  The driver uses the
serializer msgport is at:
http://leaf.dragonflybsd.org/~sephe/iwl.tbz

Best Regards,
sephe

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