fixes #124 clear buffers when drop is called

op_jack_drop sets a flag and waits till buffers are dropped in
op_jack_cb. After that it returns control to cmus.

Also the under-run code is made unnecessary by just pausing for one
period (~0.02 seconds with jack params -p 1024 -r 44100) when there is
not enough data in the buffer. Recovery is faster that way. There are
less audible artifacts and the code is prettier.
coding style: pointers got to identifier

see #124
coding style: add #if HAVE_CONFIG for config/*.h
remove debug print of jack status

size is unknown and it is of no real use here
remove struct cfg
make pause, drop and drop_done bool
Remove unnecessary cast to char*
Move temporary allocation out of loop
jack_get_buffer_size returns jack_nframes_t
Fix race condition

op_jack_drop might be called before it is finished, in that case just
return.
Merge branch 'jack-drop' of https://github.com/nefthy/cmus into nefthy-jack-drop

Conflicts:
	jack.c
jack: make server_name static