Get rid of E_SEND.

This patch also simplifies sendall() a bit and fixes two bugs in
add_one_audio_file():

	- If the check for existence of a hash sister failed, we missed
	  to unmap the audio file.
	- If an error occurred we send out a message to the client. If
	  this message can not be sent (because the client terminated
	  the connection) we returned success anyway and happily
	  tried to add the next audio file.

Some documentation improvements as well.