Add a general file-level locking API

Add a general file-level locking API

Add an API for acquiring and releasing file-level locks. This abstracts
away the platform-specific aspects of file locking, and also makes the
locking work when several threads attempt to acquire a lock within the
same process.

This commit just adds the functions and datatypes; nothing actually uses
them. The users will be added in (hopefully nearby) future commits.

Change-Id: I6bffb6a7dd47ffb5c8a9942b16cd5ef6bc7eb62e
Reviewed-on: http://gerrit.openafs.org/1237
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>