PATH- and completion-related fixes in Eshell on MS-Windows

PATH- and completion-related fixes in Eshell on MS-Windows

* lisp/eshell/esh-ext.el (eshell-search-path): When running on
MS-Windows, prepend "." to list of directories produced from PATH,
as Windows always implicitly searches the current directory first.
(eshell-force-execution): Make it have a non-nil default value on
MS-Windows and MS-DOS.
* lisp/eshell/em-cmpl.el (eshell-complete-commands-list): If
eshell-force-execution is non-nil, complete on readable files and
directories, not only executables.  When running on MS-Windows,
prepend "." to list of directories produced from PATH, as Windows
always implicitly searches the current directory first.