nrelease - Minor fixes

- Use 'reinstallkernel' to avoid leaving *.old files
  in the ISO image.
- Make sure the '/boot/kernel/kernel.BOOTP' symbolic link
  is recreated when it already exists (-f option to ln).
- Do not use '-p' to cp. In the case /etc/resolv.conf file
  has schg flag set, a later rm during nrelease would fail.

Submitted-by: YONETANI Tomokazu <y0netan1@dragonflybsd.org>
DragonFly-bug: <http://bugs.dragonflybsd.org/issues/1496>

This commit closes #1496
kernel - Correct second edge-case for machdep.pmap_mmu_optimize

* The pmap_release*() code was previously optimized to not invalidate
  on pml4, pdp, pd, or pt pte's, only on terminal ptes.  This invalidation
  interferes with the PT replacement that the pmap_mmu_optimize feature
  uses.

* The bug generally would only occur when memory is mmap()'d and munmap()'d
  multiple times, which something like postgres will not do, but which
  rtorrent and other programs will do.

* Correctly invalidate page tables (use invltlb) when PT pte's are replaced.

* Requires further testing.  Default for this feature is still off.
Remove usbdevs_data.h in /usr/include via 'make upgrade'.
Gnargh, fix typo.