gdb

gdb
	PR gdb/1815:
	* cli/cli-decode.c (delete_cmd): Forward declare.
	(delete_cmd): Now static.  Change return type.  Remove command
	from alias chain.  Rewrite.
	(add_cmd): Initialize new fields.  Update cmd_pointer on all
	aliases.
	(add_alias_cmd): Put command on alias chain.
	* command.h (delete_cmd): Don't declare.
	* cli/cli-decode.h (delete_cmd): Don't declare.
	(struct cmd_list_element) <aliases, alias_chain>: New fields.
gdb/testsuite
	* gdb.base/commands.exp (redefine_backtrace_test): New proc.
	Call it.