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

--- a/tests/test-help.t	Sun Feb 25 11:16:09 2018 -0800
+++ b/tests/test-help.t	Tue Feb 27 15:47:44 2018 -0800
@@ -967,6 +967,7 @@
    debugrename   dump rename information
    debugrevlog   show data and statistics about a revlog
    debugrevspec  parse and apply a revision specification
+   debugserve    run a server with advanced settings
    debugsetparents
                  manually set the parents of the current working directory
    debugssl      test a secure connection to a server