raop: Discard audio packets on EAGAIN

This patch modifies UDP audio packet send path so that it silently
discards audio packets which can not be sent due to blocking of the
audio streaming port.
rtp: Introduce pa_rtsp_exec_ready()

In the current RTSP implementation, there is a vulnerable window
between the RTSP object creation and the URL initialization.
If any RTSP command is issued during this period, it will lead to
crash by assertion violation.

This patch introduces pa_rtsp_exec_ready(), which returns if it is
safe to issue RTSP commands.
raop: Issue volume change request only when safe

This patch resolves the crash issue when the GNOME volume slider is
used during music is not being played back.
raop: Calculate more reasonable volume for UDP protocol

This patch tries to calculate more reasonable volume for the UDP
protocol.