raop: Merge TCP and UDP code paths + refactoring

raop: Merge TCP and UDP code paths + refactoring

TCP and UDP implementation are following two diffrent code path while code
logic is quite the same. This patch merges both code path into a unique one
and, thus, leads to a big refactoring. Major changes include:
  - moving sink implementation to a separate file (raop-sink.c)
  - move raop-sink.c protocol specific code to raop-client.c
  - modernise RTSP session handling in TCP mode
  - reduce code duplications between TCP and UDP modes
  - introduce authentication support
  - TCP mode does not constantly send silent audio anymore

About authentication: OPTIONS is now issued when the sink is preliminary
loaded. Client authentication appends at that time and credential is kept
for the whole sink lifetime. Later RTSP connection will thus look like this:
ANNOUNCE > 200 OK > SETUP > 200 OK > RECORD > 200 OK (no more OPTIONS). This
behaviour is similar to iTunes one.

Also this patch includes file name changes to match Pulseaudio naming
rules, as most of pulseaudio source code files seem to be using '-'
instead of '_' as a word separator.