Fix at least one source of the continued 'NFS append race'. close()

Fix at least one source of the continued 'NFS append race'.  close()
    was calling nfs_flush() and then clearing the NMODIFIED bit.  This is
    not legal since there might still be dirty buffers after the nfs_flush
    (for example, pending commits).  The clearing of this bit in turn prevented
    a necessary vinvalbuf() from occuring leaving left over dirty buffers
    even after truncating the file in a new operation.  The fix is to
    simply not clear NMODIFIED.

    Also added a sysctl vfs.nfs.nfsv3_commit_on_close which, if set to 1,
    will cause close() to do a stage 1 write AND a stage 2 commit
    synchronously.  By default only the stage 1 write is done synchronously.

Reviewed by:	Alfred Perlstein <bright@wintelcom.net>