Add Multipath TCP (MPTCP) support for server and client

Snapcast currently relies on a single TCP path for the audio stream. When that path dies (e.g. a WiFi link goes away), the client notices only after the TCP timeout and reconnects — an audible interruption.

Multipath TCP (MPTCP, Linux kernels >= 5.6) lets one connection use several network paths: with two links (two WLANs, WiFi + mobile), the stream survives the loss of one path without any reconnect. This is implemented and measured in a PR (to be linked).

Measured with an automated netns-based failover lab (server and client on two parallel veth links, silent link failure):

• default MPTCP configuration: full stream rate resumes in ~15-45 ms, the client stays connected with 0 errors
• plain TCP baseline: no failover at all — audio stalls until the link returns

Only the Linux build is affected; on other platforms (and on kernels without MPTCP) everything falls back to plain TCP.