Revert function pointers for rl_complete() and rl_list_possib() introduced in 0.2.2.

Instead merge afd8b4de9dca8ec6afc3 from http://github.com/heimdal/heimdal.git project.
This lets rl_complete() and rl_list_possib() become wrapper functions calling a set of
function pointers, set using rl_set_complete_func() and rl_set_list_possib_funct().

Each wrapper has a fallback to do filename completion, which in turn can be disabled
by leaving out --enable-default-complete from the configure line.

This change, admittedly quite intrusive for a library, is a better implementation in
many ways.  For one it is much more readable, but it also enables further adoption of
other editline forks as well as a simpler implementation of GNU Readline function
pointers rl_completion_entry_function and rl_attempted_completion_function at a later
stage.

My apologies to everyone for whom this change breaks backwards compatibility.  For
help on converting your code, please see examples/cli.c.

include/editline.h

@@ -21,12 +21,18 @@
 #ifndef __EDITLINE_H__
 #define __EDITLINE_H__
 
+/* Editline specific types, despite rl_ prefix.  From Heimdal project. */
+typedef char* (*rl_complete_func_t)(char*, int*);
+typedef int (*rl_list_possib_func_t)(char*, char***);
+
 /* Display print 8-bit chars as `M-x' or as the actual 8-bit char?  (Default:1) */
 extern int rl_meta_chars;
 
-/* Assign these to get command completion, see cli.c for example usage. */
-extern char *(*rl_complete)   (char *token, int *match);
-extern int   (*rl_list_possib)(char *token, char ***av);
+/* Use these functions to set custom command/file completion, see cli.c for example usage. */
+rl_complete_func_t    rl_set_complete_func(rl_complete_func_t func);
+rl_list_possib_func_t rl_set_list_possib_func(rl_list_possib_func_t func);
+
+/* Editline specific functions. */
 
 /* For compatibility with FSF readline. */
 extern int         rl_point;