child_process: use full path for cmd.exe on Win32

Currently child_process.exec() assumes that cmd.exe is on the PATH,
and fails with a spawn ENOENT error if it is not.

The Windows 'comspec' environment variable contains the full filepath
to the default command interpreter, eg "C:\Windows\System32\cmd.exe".
Should it not be set, we fall-back to using 'cmd.exe' from PATH, as
before.

Signed-off-by: Fedor Indutny <fedor@indutny.com>