Windows: fix parameters and return value from nt_seek

Windows: fix parameters and return value from nt_seek

SetFilePointerEx takes specific values
(FILE_BEGIN/FILE_CURRENT/FILE_END) whilse fseek requires SEEK_SET,
SEK_END, SEEK_CUR.  It turns out that these overlap, but we should
not let that pass unchallenged.

SetFilePointerEx returns nonzero for success zero for failure. fseek
returns the other way around.

Neither of these changes currently matter, but we should fix them.

Reviewed-on: http://gerrit.openafs.org/3746
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 4f838b03bbf9ea0e1ede8a188ea6dde3efb4e231)

Change-Id: I295d172e8f14bc50facf38f08e3dc3af02b7404b
Reviewed-on: http://gerrit.openafs.org/3864
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>