rdpsnd server: add support for clients version < 6

MS-RDPEA 2.2.2.3 states (Appendix A <6>) that quality mode PDU
should be ignored if the client version is < 6. For these clients the sound
channel got never activated since activated was only called after
reception of the quality mode PDU.

For clients < version 6 activated is now called after
CLIENT_AUDIO_VERSION_AND_FORMATS was received.
rdpsnd server: seal stream before processing

If the stream is not sealed Stream_GetRemainingLength might return bogus
values.
rdpsnd_server_handle_messages: change return type

With BOOL as return type it is not possible to differentiate between
success and "no data can be read" (when a channel read would block).

rdpsnd_server_handle_messages returns now int with the following
possible values:

-1 if no data could be read
0 error (like connection close) (formerly FALSE)
1 succsess (also if further bytes need to be read) (formerly TRUE)

Not using -1 for error cases was chosen to be compatible with the BOOL
return values used before.
Merge pull request #2088 from bmiklautz/fix/rdpsndv5

rdpsnd server side: misc fixes