Windows [find_executable] fixes

[jonahbeckford]

• Use ';' as PATH seperator on Windows.

• On Windows Sys.file_exists can return true even if Sys.is_directory is true. And an executable ends in .exe

Before

utop # eval (find_executable_exn "git") ;;
Exception: Failure "command \"git\" not found".
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Shexp_process__Process.map.(fun) in file "process-lib/src/process.ml", line 533, characters 40-45
Called from Shexp_process__Process.exec in file "process-lib/src/process.ml", line 173, characters 12-47
Re-raised at Shexp_process__Process.exec in file "process-lib/src/process.ml", line 176, characters 8-19
Called from Stdlib__Fun.protect in file "fun.ml", line 33, characters 8-15
Re-raised at Stdlib__Fun.protect in file "fun.ml", line 38, characters 6-52
Called from Topeval.load_lambda in file "toplevel/byte/topeval.ml", line 89, characters 4-153

And it has the same exception for git.exe.

After

utop # let open Shexp_process in eval (find_executable "git" ) ;;
- : string option = Some "C:\\Program Files\\Git\\cmd\\git.exe"

utop # let open Shexp_process in eval (find_executable "git.exe" ) ;;
- : string option = Some "C:\\Program Files\\Git\\cmd\\git.exe"

[mdelvecchio-jsc]

A couple notes:

• Sys.file_exists also returns true on unix; why should we special-case it for windows?
• Finding an executable from the path in windows involves checking the PATHEXT environment variable to determine what extensions to look for. I think it would be bad to "support" windows but not correctly use this environment variable.
• The mli for this function explicitly says that it uses a unix environment. I think we should either 1) provide proper support for windows here (which involves some design questions that I don't quite have a good answer to); or 2) just leave the implementation as is (maybe change the name to make it more obvious it's unix-only?).