Linux: Add autoconf macro for structure checks

Linux: Add autoconf macro for structure checks

Add a new autoconf macro for doing structure element checks, and
modify all simple structure checks to use it. This introduces a
standard name form - STRUCT_structure_HAS_element, so there are
some changes in the code to make use of this standard form.

Change-Id: Ife967322503ae6f428e76845000de04f26929e65
Reviewed-on: http://gerrit.openafs.org/1853
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>