Clean up "completer_handle_brkchars" callback handling

Clean up "completer_handle_brkchars" callback handling

This patch cleans up "completer_handle_brkchars" callback handling:

- Renames the function typedef to better match its intent:
  completer_ftype_void ->  completer_handle_brkchars_ftype

- Factors out common code in complete_line_internal handling the
  "handle_brkchars" callback to a separate function.

- Centralizes all the "completer method" to "handle_brkchars method"
  mapping in a single function.

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* cli/cli-decode.c (set_cmd_completer_handle_brkchars): Adjust to
	renames.
	* cli/cli-decode.h (struct cmd_list_element) <completer>: Move
	comments to completer_ftype's declaration.
	<completer_handle_brkchars>: Change type to
	completer_handle_brkchars_ftype.
	* command.h (completer_ftype): Add describing comment and give
	names to parameters.
	(completer_ftype_void): Rename to ...
	(completer_handle_brkchars_ftype) ... this.  Add describing comment.
	(set_cmd_completer_handle_brkchars): Adjust.
	* completer.c (filename_completer_handle_brkchars): New function.
	(complete_line_internal_normal_command): New function, factored
	out from ...
	(complete_line_internal): ... here.
	(command_completer_handle_brkchars)
	(default_completer_handle_brkchars)
	(completer_handle_brkchars_func_for_completer): New functions.
	* completer.h (set_gdb_completion_word_break_characters): Delete
	declaration.
	(completer_handle_brkchars_func_for_completer): New declaration.
	* python/py-cmd.c (cmdpy_completer_handle_brkchars): Adjust to use
	completer_handle_brkchars_func_for_completer.