Issue #19095: SSLSocket.getpeercert() now raises ValueError when the SSL handshake hasn't been done.

Patch by Xavier de Gaye.

Au_read.readframes() now updates current file position and reads correct
number of frames from multichannel stream.  Au_write.writeframesraw() now
correctly updates current file position.  Au_read.getnframes() now returns an
integer (as in Python 2).  Au_read and Au_write now correctly works with file
object if start file position is not a zero.