serve: add and use portable spawnvp replacement There is no standard python command to really detach a process under Windows. Instead we use the low level API wrapped by subprocess module with all necessary options to avoid any kind of context inheritance. Unfortunately, this version still opens a new window for the child process. The following have been tried: - os.spawnv(os.P_NOWAIT): works but the child process is killed when parent console terminates. - os.spawnv(os.P_DETACH): works on python25, hang on python26 when writing to the hgweb output socket. - subprocess.CreateProcess() hack without shell mode: similar to os.spawnv(os.P_DETACH). Fix 1/3 for issue421