nfscl: Add support for va_birthtime to NFSv4

There is a NFSv4 file attribute called TimeCreate
that can be used for va_birthtime.
r362175 added some support for use of TimeCreate.
This patch completes support of va_birthtime by adding
support for setting this attribute to the server.
It also eanbles the client to
acquire and set the attribute for a NFSv4
server that supports the attribute.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30156