gdb: add cmd_list_element::is_command_class_help

gdb: add cmd_list_element::is_command_class_help

Same idea as the previous patches, but for whether a command is a
"command class help" command.  I think this one is particularly useful,
because it's not obvious when reading code what "c->func == NULL" means.

Remove the cmd_func_p function, which does kind of the same thing as
cmd_list_element::is_command_class_help (except it doesn't give a clue
about the semantic of a NULL func value).

gdb/ChangeLog:

	* cli/cli-decode.h (cmd_list_element) <is_command_class_help>:
	New, use it.
	* command.h (cmd_func_p): Remove.
	* cli/cli-decode.c (cmd_func_p): Remove.

Change-Id: I521a3e1896dc93a5babe1493d18f5eb071e1b3b7