refactor(proxy): keep the proxy state files behind one module

1. Proxy state lived in marker files read and written by copy-pasted code in several places, so clean missed proxy.unprivileged, a second proxy start -p overwrote proxy.pid, and an unprivileged proxy forgot HTTPS on restart.
2. ProxyState now owns every proxy.* file and the one list clean uses, and the three bugs are fixed in it.
3. Fixes #521. Fixes #564. Builds on #570.

Co-authored by @dibas-np, based on #540. Co-authored by @Knat-Dev, based on #566. Co-authored by @jasonkuhrt, based on #536.