Build afsio on Windows; remove many warnings

Build afsio on Windows; remove many warnings

With this commit afsio builds on Windows.   It will not work
until the VIOC_FS_CMD OSD extension is implemented in the
Windows Cache Manager.

Required changes:

  * Export ugen_ClientInit from libafsauthent.dll

  * Fix util_GetInt64 and util_GetUInt64 to return afs_int32
    instead of afs_int64 since it is just returning success
    or error

  * Define VIOC_FS_CMD for Windows even though it isn't implemented

LICENSE BSD

Reviewed-on: http://gerrit.openafs.org/587
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>