Don't use short file name for nonexistent exes

Avoid the temptation to handle missing exes specially in the win32 case.

* el-get-core.el (el-get-shell-quote-program): If we can't executable-find
  the program, just shell-quote-argument it (as in non-Windows case).
quote shell args only if needed

Windows' CreateProcess() function does weird things when given quoted
args, therefore avoid quoting when possible.

* el-get-core.el (el-get-no-shell-quote): New constant.
(el-get-shell-quote-program): Don't quote if program-name matches
el-get-no-shell-quote.
(el-get-maybe-shell-quote-argument): New function.
(el-get-start-process-list): Use it.