Added support for environment variable specification of parameters.

SCYTHERPARALLELLN
  Worker count. Set to 0 to use cpu_count()

SCYTHERPARALLELLM
  Prefix filter modulus. Sensible values are 2 and 4. If it is 0, nothing gets checked.

SCYTHERPARALLELLL
  Prefix filter sequence length. (Note triple L at end of key)