hammer2 - freemap part 3 - group by allocation size

* Each freemap leaf represents ~2MB worth of storage.  Assign a radix to
  each leaf, limiting allocations from that leaf to that radix.

  This primarily results in inodes being grouped together, improving
  the performance for find, ls or other topological scans.  We could
  improve this but for now we'll stick with it as-is.

  This mechanic also allows us to use cluster_read().  This function is
  used for everything except volume-header and freemap elements.

* More formally handle logical sizes vs allocation sizes vs device I/O
  sizes.  For example, a 1KB inode allocates 1KB using 16KB device I/O's.

* Beef up the sysctl I/O counters.
hammer2 - Add 'hammer2 freemap' directive

* Beef up 'show' and add the 'hammer2 freemap' directive to dump the
  freemap.
hammer2 - Adjust newfs_hammer2 for recent media changes

* Adjust newfs_hammer2 for recent media changes.  The freemap is now based
  in the volume header with a blockref set instead of a single blockref.
Merge branch 'master' into gsocdrm_alter
/usr/Makefile: Add pkg-bootstrap target

New snapshots provide a pre-built "pkg" tool so pre-build dports
binaries can be installed easily.  However, upgrades from older releases
won't have "pkg" available, and for those systems they had to download
the entire dports repository just to build pkg in order to take advantage
of the available pre-built packages.

To fix this situation, and the situation where /usr/local/sbin/pkg is
lost for any reason, a new make target has been added to /usr/Makefile:
pkg-bootstrap

The pkg-bootstrap target will download a pre-built "pkg-static" program
along with pkg.conf and all the man pages.  In reality, pkg-static is
only used for one command, and that is to install a full "pkg" program
from the dragonfly repository.

If "pkg.conf" already exists, a message will instruct the user to move
it first.  If /usr/local/sbin/pkg already exists, the target won't work
and it won't even show as an option.

After pkg-static is installed, the user will be instructed to type
"rehash; pkg-static install -y pkg; rehash" which should result in the
system having the latest pkg on their system and thus can proceed to
install packages normally.
Merge branch 'master' into gsocdrm_merger