Add a global vnode cache:

- vcache_get() retrieves a referenced and initialised vnode / fs node pair.
- vcache_remove() removes a vnode / fs node pair from the cache.

On cache miss vcache_get() calls new vfs operation vfs_loadvnode() to
initialise a vnode / fs node pair.  This call is guaranteed exclusive,
no other thread will try to load this vnode / fs node pair.

Convert ufs/ext2fs, ufs/ffs and ufs/mfs to use this interface.

Remove now unused ufs/ufs_ihash

Discussed on tech-kern.

Welcome to 6.99.41
Use TOOL_NCDCS to turn an ibmnws kernel automatically into a valid NCD
firmware image for netbooting. This is still a valid ELF kernel, but with
a patched CRC checksum and image size included.
Revert default strategy change from 1.298 (which has since moved).

Reverting to "fcfs" nearly doubles the speed of sequential reads from
a level-1 RAID (previously using the default of "priocscan").

Needs pullup via patch for netbsd-6.