MINOR: connection: rename QMux related flags

MINOR: connection: rename QMux related flags

This is a follow-up on the QUIC MUX renaming process.

The current patch performs renaming of "qstrm" to "qmux" in connection
flags. These flags are only used in linked with the xprt_qmux layer.
This has an impact on every files which manipulates these flags, namely
backend, session and ssl_sock sources.

Also, internal xprt identifier is renamed from XPRT_QSTRM to XPRT_QMUX,