namei: Create the IH_CREATE_INIT function

Create a new function that combines calls to IH_CREATE and IH_INIT
into one operation; the new function is called IH_CREATE_INIT. This
allows a caller to create a file and then use it, without needing to
open() the file twice.

This is currently only implemented for the Unix namei backend; other
backends result in effectively the same functionality (but can use the
same API).

Change-Id: I93d531a9892beeb0c1ceac18458cbe0f1e3a0ded
Reviewed-on: http://gerrit.openafs.org/9969
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>