Find the BitTorrent dissector immediately after registering it, and save

Find the BitTorrent dissector immediately after registering it, and save
the handle, rather than finding it every time we need it.

Just make a guint8 array with the BitTorrent magic string length
followed by the string, and use "tvb_memeql()" to test against that - it
handles checking to make sure there's enough data in the tvbuff (if
not, it returns -1 as a "no match" indication) and then checks all of
the first 20 bytes in one operation.

Clean up white space.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@14882 f5534014-38df-0310-8fa8-9805f1628bb7