This was a mistake, it should definitely be using the device namespace
rather than the file namespace. As it says in the docs, all pipe names
must start with \\.\pipe\
If the program name isn't quoted and the .exe it refers to isn't found,
CreateProcess will add the program arguments to the program name and
continue searching, so for "program arg1 arg2", CreateProcess would try
"program.exe", "program arg1.exe", then "program arg1 arg2.exe". This
behaviour is weird and not really desirable, so prevent it by always
quoting the program name.
When quoting argv[0], escape sequences shouldn't be used. msvcrt, .NET
and CommandLineToArgvW all treat argv[0] literally and end it on the
trailing quote, without processing escape sequences.
If the stdout or stderr write callback is NULL, then don't redirect this
stream. Preparation for the next commit.
Not sure what to do on Windows; it seems STARTUPINFO doesn't allow
redirection only one of them. So just let them write nothing. For our
intended use-case (next commit), this is probably sensible.
The subprocess code was already split into fairly general functions,
separate from the Lua code. It's getting pretty big though, especially
the Windows-specific parts, so move it into its own files.
