{ctype,iswalnum}_l.3: Remove references to non-existant is{w,}ascii_l().
kernel - Fix rare buffer cache deadlock

* cluster_collectbufs() was improperly using a blocking vfs/bio calls
  to find nearby buffers, which can deadlock against multi-threaded
  filesystems.

* Only occurs in the write path, probably only H2 is affected.
kernel - Add cache_isopen()

* Add cache_isopen(), a function which nominally returns whether the
  file referenced by a namecache structure is active (opened or mmapped).

  The code currently just checks the ref-count on the underlying vnode
  if it is present, which should be sufficient.

* Used by HAMMER2.