gdb: rename cmd_list_element::prefixlist to subcommands

gdb: rename cmd_list_element::prefixlist to subcommands

While browsing this code, I found the name "prefixlist" really
confusing.  I kept reading it as "list of prefixes".  Which it isn't:
it's a list of sub-commands, for a prefix command.  I think that
renaming it to "subcommands" would make things clearer.

gdb/ChangeLog:

	* Rename "prefixlist" parameters to "subcommands" throughout.
	* cli/cli-decode.h (cmd_list_element) <prefixlist>: Rename to...
	<subcommands>: ... this.
	* cli/cli-decode.c (lookup_cmd_for_prefixlist): Rename to...
	(lookup_cmd_with_subcommands): ... this.

Change-Id: I150da10d03052c2420aa5b0dee41f422e2a97928