Allow compiling with KERNEL and AFS_PTHREAD_ENV

Allow compiling with KERNEL and AFS_PTHREAD_ENV

Add the necessary preprocessor conditionals to allow building
libuafs with AFS_PTHREAD_ENV defined.

A follow-up commit will switch to building libuafs using libtool,
which will set the pthread compiler/linker flags.  UKERNEL is already
using the pthread primitives for its internal kernel synchronization,
so there should not be any harm from additionally specifying the
pthread build arguments.

This change was produced mostly in a mechanical fashion, attempting
to perform such a build, and eliminating compiler and linker errors
in an iterative process.  No concerted effort has been made to audit
the whole kernel codebase for correctness of conditionals, but the
linktest executable does link (that is, the overall build succeeds).

Change-Id: I14a3ab5fce72812d92ba5657c734783dbd086ee3
Reviewed-on: http://gerrit.openafs.org/11546
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>