File extraction tweaks.

- Default extraction limit of 100MB now provided via a tuning script
  loaded in local.bro so that command-line Bro is unlimited by default.

- Extraction directory is now created on request of file extraction
  rather than unconditionally in bro_init().
Fix wrong documentation for mkdir BIF.
Remove code relict pointed out by Bernhard.

The condition should never be true, it's leftover from my
hacking/debugging code.
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Remove code relict pointed out by Bernhard.
  Fix wrong documentation for mkdir BIF.
  File extraction tweaks.