MFC r266208: Speed up pmcstat by improving string hash

  In one case generating callgraph output from a 24MB system-wide sampling
  data file took 17.4 seconds on average.  Profiling showed pmcstat
  spending a lot of time in strcmp, due to hash collisions.

  Replacing the XOR-only hash with FNV-1a reduces the run time for my
  test by 40%.
MFC r259107
  Eliminate a redundant parameter to vm_radix_replace().

  Improve the wording of the comment describing vm_radix_replace().