(parse_forward_arg): Return targret address on success. Don't
modify the input string, using updated parse_arg_unsigned. Don't
use io_lookup_address; names are resolved later.
(main_argp_parser): Use make_string. Updated to parse_forward_arg
changes. When setting up tcp forwarding, use "" to denote any
address, and "localhost" to denote any local addresses.

Rev: src/lsh.c:1.224