bug#21323: start-process rejects .com executables

[Eli Zaretskii]

> Date: Sat, 22 Aug 2015 19:24:00 +0200
> From: Björn Lindqvist <bjourne@gmail.com>
> 
> I found this bug on windows ver 24.5.1. If you try to run a .com file:
> 
> (start-process "X" "Y" "c:/Windows/System32/tree.com")
> 
> You get: "Spawning child process: exec format error"

Indeed.

> The cryptic error appears to be a result of the .com extension.

No, it's because .com programs are run via cmdproxy.exe, and there's a
bug in computing the directory where we look for it.

As a workaround, you can set CMDPROXY in the environment to point to
cmdproxy.exe (should be in libexec/emacs/24.5/i686-pc-mingw32/
subdirectory of the directory where you installed Emacs 24.5).

I've now fixed this for the next release so it will work even without
setting CMDPROXY in the environment.  Thanks.