[cmyth] Implement cmyth_recorder_get_frames_written

Implement the previously empty stub cmyth_recorder_get_frames_written.
The function queries the number of frames the recorder has written to
disk.

Currently this function is implemented for protocol version >=66.
[cmyth] Implement cmyth_rcv_keyframe

This implements the previously empty stub for cmyth_rcv_keyframe.
The function reads a key frame from the socket.
[cmyth] Add cmyth_rcv_posmap

Adds function cmyth_rcv_posmap. The function reads the position map
from the socket.
[cmyth] Implement cmyth_recorder_get_keyframe_pos

This implements the previously empty function
cmyth_recorder_get_keyframe_pos. that queries the byte location of the
next key frame after the position given.

The function is currently implemented for backend version >=66.
[cmyth] Implement cmyth_recorder_get_position_map

This implements the previously empty stub
cmyth_recorder_get_position_map that queries the position map from the
backend.

The function is currently implemented for backend version >=43.
[cmyth] Add cmyth_recorder_get_duration_map

This adds the function cmyth_recorder_get_duration_map that queries
the duration map from the backend.

The function is currently implemented for backend version >=77.
[cmyth] Add cmyth_keyframe_number
[cmyth] Add cmyth_keyframe_pos
[cmyth] Add cmyth_posmap_count
[cmyth] Add cmyth_posmap_keyframe