Fix path for current directory in eshell on MS-Windows

Fix path for current directory in eshell on MS-Windows

On MS-Windows, PATH implicitly includes the current directory.
Do it right for Eshell by adding "./" instead of ".", to
avoid finding .FOO instead of ./FOO.
* lisp/eshell/esh-util.el (eshell-get-path): New function.
* lisp/eshell/em-cmpl.el (eshell-complete-commands-list):
* lisp/eshell/esh-ext.el (eshell-search-path): Use eshell-get-path.