Removed msg_change_fec_data, use msg_universal instead.

This is a more versatile solution allowing further extensions without
a need to modify the global structure at an expese of worse type control.
As a replacement, a tag starting msg_universal::text is used/suggested.

1. set the volume for all channels
2. correctly set attribute muted according to the value (until now it
   was actually never set)

- show actual playback volume as 0 (-inf) if muted receiver
- display explicitly receiver mute status
- "Received audio volume" -> "Playback volume" - this has more clear
  semantics and is consistent with message written when '*'/'/'/'m' is
  pressed.

Do not show hint to use NAT traversal if user uses private or loopback
address as a peer address.

This avoids misleading displaying in otherwise legitimate cases -
transmissions over loopback or behind NAT but just inside LAN.

Use UINT32_MAX for sizes first. This may help if WAV file is incorrectly
closed indicating that the size values aren't valid (at least FFmpeg
understands it so).

+ fixed message for overflow

This modernizes the code and makes the semantic of prototypes more clear.

Enable querying location of executable

-add function get_exec_path and C++ wrapper around it

Do not assume that initialization of null device must succeed - eg. in
some errorneous setup there may be API mismatch leading to init failed.

Was printed NULL ("(null)") because *fmt was set to NULL early.

[rtp] fix display_buf_increase_warning warning

rtp_set_recv_buf returns TRUE on success, which is #define'd to 1. Fix the logic in the check so the warning does not print erroneously.

SDL cflags define '-Dmain=SDL_main' otherwise leading to wrong main func
signature (perhaps SDL.h has some magic that corrects that but it isn't
included by our main.cpp).

FIX: SDL/SDL2 includes were missing in module CXXFLAGS, causing incompatibility on Ubuntu 20.04 with clang-10

Avoid needless seeks - data chunk must follow fmt chunk [1] thus the
file can be read without seeking (without the file size check).

[1] http://www.tactilemedia.com/info/MCI_Control_Info.html

Thread was not correctly stopped causing crashes.

Shouldn't make a difference but just to be consistent.

Similarly to WAV reader write UINT32_MAX as a placeholder if file is
longer than 4 GiB

Has data chunk size set to UINT32_MAX and size is deduced from file
length. This is consistent to what FFmpeg (MediaInfo, MPV and perhaps
others) does.

For debugging only.

GCC complains on that with -Wattributes