when there is no memory available to do so. Use the uvnode's strategy
routine to retrieve data from the device, rather than always using pread().
Add a buffer header flag that specifies external management of the buffer's
data area.
All of this in support of a new cleaner, which is not included in this commit.
* Extend the lfs library from fsck_lfs(8) so that it can be used with a
not-yet-existent LFS. Make newfs_lfs(8) use this library, so it can
create LFSs whose Ifile is larger than one segment.
* Make newfs_lfs(8) use strsuftoi64() for its arguments, a la newfs(8).
* Make fsck_lfs(8) respect the "file system is clean" flag.
* Don't let fsck_lfs(8) think it has dirty blocks when invoked with the
-n flag.
