gdb: make skip_over_slash_fmt available outside printcmd.c

Move the function skip_over_slash_fmt into completer.c, and make it
extern, with a declaration in completer.h.

This is a refactor in order to support the next commit.  I've not
changed any of the code in skip_over_slash_fmt.

There should be no user visible changes after this commit.

gdb/printcmd.c

@@ -1380,71 +1380,6 @@ print_command_1 (const char *args, int voidprint)
     }
 }
 
-/* Called from command completion function to skip over /FMT
-   specifications, allowing the rest of the line to be completed.  Returns
-   true if the /FMT is at the end of the current line and there is nothing
-   left to complete, otherwise false is returned.
-
-   In either case *ARGS can be updated to point after any part of /FMT that
-   is present.
-
-   This function is designed so that trying to complete '/' will offer no
-   completions, the user needs to insert the format specification
-   themselves.  Trying to complete '/FMT' (where FMT is any non-empty set
-   of alpha-numeric characters) will cause readline to insert a single
-   space, setting the user up to enter the expression.  */
-
-static bool
-skip_over_slash_fmt (completion_tracker &tracker, const char **args)
-{
-  const char *text = *args;
-
-  if (text[0] == '/')
-    {
-      bool in_fmt;
-      tracker.set_use_custom_word_point (true);
-
-      if (text[1] == '\0')
-	{
-	  /* The user tried to complete after typing just the '/' character
-	     of the /FMT string.  Step the completer past the '/', but we
-	     don't offer any completions.  */
-	  in_fmt = true;
-	  ++text;
-	}
-      else
-	{
-	  /* The user has typed some characters after the '/', we assume
-	     this is a complete /FMT string, first skip over it.  */
-	  text = skip_to_space (text);
-
-	  if (*text == '\0')
-	    {
-	      /* We're at the end of the input string.  The user has typed
-		 '/FMT' and asked for a completion.  Push an empty
-		 completion string, this will cause readline to insert a
-		 space so the user now has '/FMT '.  */
-	      in_fmt = true;
-	      tracker.add_completion (make_unique_xstrdup (text));
-	    }
-	  else
-	    {
-	      /* The user has already typed things after the /FMT, skip the
-		 whitespace and return false.  Whoever called this function
-		 should then try to complete what comes next.  */
-	      in_fmt = false;
-	      text = skip_spaces (text);
-	    }
-	}
-
-      tracker.advance_custom_word_point_by (text - *args);
-      *args = text;
-      return in_fmt;
-    }
-
-  return false;
-}
-
 /* See valprint.h.  */
 
 void