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).

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>
(cherry picked from commit aaa2584c0498037529a30c985ac8a437ec7d842c)

Change-Id: I5018418cea63ba19ef0ef3bb047e3f9799a93293
Reviewed-on: http://gerrit.openafs.org/10177
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>