-Wwrite-strings: Constify word break character arrays

-Wwrite-strings flags several cases of missing casts around
initializations like:

   static char *gdb_completer_command_word_break_characters =
    " \t\n!@#$%^&*()+=|~`}{[]\"';:?/>.<,";

Obviously these could/should be const.  However, while at it, there's
no need for these variables to be pointers instead of arrays.  They
are never changed to point to anything else.

Unfortunately, readline's rl_completer_word_break_characters is
"char *", not "const char *".  So we always need a cast somewhere.  The
approach taken here is to add a new
set_rl_completer_word_break_characters function that becomes the only
place that writes to rl_completer_word_break_characters, and thus the
single place that needs the cast.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* ada-lang.c (ada_completer_word_break_characters): Now a const
	array.
	(ada_get_gdb_completer_word_break_characters): Constify.
	* completer.c (gdb_completer_command_word_break_characters)
	(gdb_completer_file_name_break_characters)
	(gdb_completer_quote_characters): Now const arrays.
	(get_gdb_completer_quote_characters): Constify.
	(set_rl_completer_word_break_characters): New function.
	(set_gdb_completion_word_break_characters)
	(complete_line_internal): Use it.
	* completer.h (get_gdb_completer_quote_characters): Constify.
	(set_rl_completer_word_break_characters): Declare.
	* f-lang.c (f_word_break_characters): Constify.
	* language.c (default_word_break_characters): Constify.
	* language.h (language_defn::la_word_break_characters): Constify.
	(default_word_break_characters): Constify.
	* top.c (init_main): Use set_rl_completer_word_break_characters.

gdb/f-lang.c

@@ -203,7 +203,7 @@ f_language_arch_info (struct gdbarch *gdbarch,
 
 /* Remove the modules separator :: from the default break list.  */
 
-static char *
+static const char *
 f_word_break_characters (void)
 {
   static char *retval;