UNIX adapter: winpty.exe searches the PATH for ./program.exe executable

[rprichard]

Here it's using ls from the PATH even though an explicit path is given:

$ ls ./ls
ls: cannot access './ls': No such file or directory

$ winpty ./ls
build      build-libpty  configure  Makefile  README.md    ship  vcbuild.bat
build-gyp  config.mk     LICENSE    misc      RELEASES.md  src   VERSION.txt

It does so even if there really is a file at the explicit path:

$ ./winpty-agent.exe --version
winpty version 0.4.0-dev
commit e0cbe70600b4cc3674f4caa3d1f2de7af752c80f

$ winpty ./winpty-agent.exe --version
winpty version 0.3.0
commit 87436ca8ae208471776964052bb62f575ecc891a

[rprichard]

Related to #6 (and maybe #8).