Bug 25.0: `parse-colon-path' deals with `//' diferently as unix systems

[Tino Calancha <f92capac@gmail.com>]

Current implementation use `substitute-in-file-name' which substitute environment variables; that 
function deal with '//' differently as expected on unix systems:

In unix systems there is no difference between:

I)  /foo//bar/baz
II) /foo/bar/baz

but `substitute-in-file-name' will translate I) into:
/bar/baz

I found more sensible if 'parse-colon-path' would made no difference
between I) and II).

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In *scratch* buffer after started session with:
emacs -Q

(let ((foo "/home/user/soft/bin/:/usr/bin/:/usr/local/bin"))
   (setenv "FOO" foo)
   (parse-colon-path "$FOO"))
("/home/user/soft/bin/:/usr/bin/:/usr/local/bin/")
(let ((foo "/home/user/soft/bin/:/usr/bin/:usr/local/bin/"))
   (setenv "FOO" foo)
   (parse-colon-path "$FOO"))
("/")

(let ((foo "/home/user/soft//bin/:/usr/bin/:/usr/local/bin"))
   (setenv "FOO" foo)
   (parse-colon-path "$FOO"))
("/bin/:/usr/bin/:/usr/local/bin/")

(let ((foo "/home/user/soft//bin/:/usr/bin/:/usr/local/bin/"))
   (setenv "FOO" foo)
   (parse-colon-path "$FOO"))
("/")

(let ((foo "/home/user/soft/bin/:/usr/bin/"))
   (setenv "FOO" foo)
   (parse-colon-path (getenv "FOO")))
("/home/user/soft/bin/" "/usr/bin/")

(let* ((foo "/home/user")
        (bar "$FOO/soft/bin/:/usr/bin/"))
   (setenv "FOO" foo)
   (setenv "BAR" bar)
   (parse-colon-path (getenv "BAR")))
("/home/user/soft/bin/" "/usr/bin/")

(let* ((foo "/home/user/")
 	   (bar "$FOO/soft/bin/:/usr/bin/"))
   (setenv "FOO" foo)
   (setenv "BAR" bar)
   (parse-colon-path (getenv "BAR")))
("/soft/bin/" "/usr/bin/")

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Emacs  : GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
  of 2015-09-06
Package: Org-mode version 8.2.10 (release_8.2.10 @ /home/calancha/tmp/emacs_git/emacs/lisp/org/)