Show info from multiple protocols when protocol is not specified

Show info from multiple protocols when protocol is not specified

Most commands like 'show ospf neighbors' fail when protocol is not
specified and there are multiple instances of given protocol type.
This is annoying in BIRD 2, as many protocols have IPv4 and IPv6
instances. The patch changes that by showing output from all protocol
instances of appropriate type.

Note that the patch also removes terminating cli_msg() call from these
commands and moves it to the common iterating code.