Restore info_command and breakpoint

Restore info_command and breakpoint

As discussed on gdb-patches, this restores info_command and the
breakpoint on info_command in gdb-gdb.gdb.  This reverts a tiny part
of 0743fc83c03 ("Replace most calls to help_list and cmd_show_list"),
as well as 652fc23a30a ("Remove gdb-gdb.gdb breakpoint on disappeared
function info_command.").

gdb/ChangeLog
2020-05-11  Tom Tromey  <tromey@adacore.com>

	* cli/cli-cmds.c (info_command): Restore.
	(_initialize_cli_cmds): Use add_prefix_command for "info".
	* gdb-gdb.gdb.in: Restore breakpoint on info_command.