make_abs_path doesn't need the directories to actually exist in the
file system.

improved error output.

make it act properly on iobuf errors.

The add and del commands adds or removes torrents from this library.
The start and stop commands are used to active or deactivate torrents.
Also, a mechanism for qeurying data on torrents has been added. It's
only used by the btcli list and stat commands yet though.

btcli has been split into different files for each command.

Both btpd and btcli now use misc/btpd_if.h for all ipc definitions.

Misc changes:
- struct metainfo is gone. Use the new mi_* functions.
- Add printf format type checking where appropriate.