debugcommands: add debugserve command `hg serve --stdio` requires the exact command argument form `hg -R <path> serve --stdio` for security reasons. An upcoming commit will need to start an SSH protocol server process with custom settings. This commit creates a `hg debugserve` command for starting servers with custom options. There are no security restrictions and we can add options here that aren't appropriate for built-in commands. We currently only support starting an SSH protocol server using the process's stdio file descriptors. The server supports logging its I/O activity to a file descriptor number passed as a command argument. Differential Revision: https://phab.mercurial-scm.org/D2464