file system. In particular, stop overwriting mount point
flags in nfs_mountdiskless() because now they are set
elsewhere. (They were _initialized_ by that function in
the 4.4BSD days, when mount structures were not allocated
in a centralized manner -- see rev. 1.1 of this file.)
Fix nfs_mount(), which happened to depend on the loss of
MNT_ROOTFS when it came to update handling.
Also note that mountnfs() no longer handles updates. Now
they shouldn't reach this function, so printf a diagnostic
message if that happens due to a coding error.