Tahoe DCC support

(Sorry it's all in one commit. I messed up badly.)
python-magic required for DCC support
Tidy up a bit; stream instead of buffer.
Do some initial buffering for magic.
Update tahoe scanning for MDMF files.

Also add error logging and disabling tahoe scanning for verifier URIs (since
there's no useful information to get out of them).
twat -> twit
Tear out the broken DCC stuff.