gdb: rename cmd_list_element::cmd_pointer to target

cmd_pointer is another field whose name I found really not clear.  Yes,
it's a pointer to a command, the type tells me that.  But what's the
relationship of that command to the current command?  This field
contains, for an alias, the command that it aliases.  So I think that
the name "alias_target" would be more appropriate.

Also, rename "old" parameters to "target" in the functions that add
aliases.

gdb/ChangeLog:

	* cli/cli-decode.h (cmd_list_element) <cmd_pointer>: Rename
	to...
	<alias_target>: ... this.
	(add_alias_cmd): Rename old to target.
	(add_info_alias): Rename old_name to target_name.
	(add_com_alias): Likewise.

Change-Id: I8db36c6dd799fae155f7acd3805f6d62d98befa9