<shadowsocks/control.h> starts ss-local on a library-owned thread,
stops it from any thread through a libuv async handle and waits for it;
<shadowsocks/shadowsocks.hpp> wraps that in a move-only, header-only
shadowsocks::LocalServer whose destructor stops and waits. The library
exports only C symbols, shadowsocks.h is unchanged, and the shared
library version becomes 2.1.0 (SOVERSION 2). test_local_api_cpp covers
the lifecycle; the installed-package consumer gains a C++ program.
