Add new "alias" command.

	Add new "alias" command.
	* NEWS: Mention new command.
	* command.h (valid_user_defined_cmd_name_p): Declare.
	* defs.h (make_cleanup_dyn_string_delete): Declare.
	* utils.c: #include "dyn-string.h".
	(do_dyn_string_delete, make_cleanup_dyn_string_delete): New functions.
	* cli/cli-cmds.c: #include "dyn-string.h".
	(argv_to_dyn_string, valid_command_p, alias_command): New functions.
	(init_cli_cmds): Add new command.
	* cli/cli-decode.c (valid_user_defined_cmd_name_p): New function.

	doc/
	* gdb.texinfo (Extending GDB): Document alias command.

	testsuite/
	* gdb.base/alias.exp: Add tests for alias command.