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.

Change-Id: Ib31cf6265fa1e714232ff0d2e099c657e41f17e9
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>