ffap: Support environment variable expansion in file names

Fixes: debbugs:19839

* lisp/ffap.el (ffap-string-at-point-mode-alist): Support
environment variable expansion in file names.

lisp/ffap.el

@@ -1004,7 +1004,7 @@ If a given RFC isn't in these then `ffap-rfc-path' is offered."
     ;; Slightly controversial decisions:
     ;; * strip trailing "@" and ":"
     ;; * no commas (good for latex)
-    (file "--:\\\\$+<>@-Z_[:alpha:]~*?" "<@" "@>;.,!:")
+    (file "--:\\\\$\\{\\}+<>@-Z_[:alpha:]~*?" "<@" "@>;.,!:")
     ;; An url, or maybe a email/news message-id:
     (url "--:=&?$+@-Z_[:alpha:]~#,%;*()!'" "^[0-9a-zA-Z]" ":;.,!?")
     ;; Find a string that does *not* contain a colon: