rid of the global btpd struct. Some fields in the struct got a corresponding
global variable whereas some was made static and moved to a module.

The bandwidht algorithm also got tweaked. It now tries to fire the event
at specific times. This was to make the code simpler. It'll probably have
to be tweaked again :P

First step to make btpd run from a directory where it'll keep the log,
socket, configuration, data on the known torrents, etc. Btpd now uses
flock on the pidfile instead of connecting to the socket in order to
be reasonably sure that no other btpd runs in the same directory.