rtspconnection: Start CSeq at 1
RFC 7826 recommends (but does not require) starting at 0, but at least one known server implementation fails to copy request sequence numbers <1 into responses due to an incorrect null check.
The server known to exhibit this behavior is the Parrot Streaming Server, serving video from their UAV devices. A fix has been submitted upstream as well: https://github.com/Parrot-Developers/librtsp/pull/2
The Parrot developers are known to have tested with LibVLC. In WireShark debugging, LibVLC appears to start with a CSeq of 2, which is likely why this bug went unnoticed.
This reverts 487595a7, which set this to 0 citing the RFC. The switch to 0 was thus a recent one; it's therefore possible server implementors relied on the previous GStreamer client behavior in their tests as well.
Fixes #624 (closed).