babel: fix crashes and improve the redistribute command

babel: fix crashes and improve the redistribute command

Fixes the following crashes:
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "no redistribute pim"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "no redistribute eigrp"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "no redistribute nhrp"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "no redistribute table"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "no redistribute vnc"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "redistribute pim"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "redistribute eigrp"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "redistribute nhrp"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "redistribute table"
babeld aborted: vtysh -c "configure terminal" -c "router babel" -c "redistribute vnc"

While here, add an option to chose if we want to redistribute IPv4 or
IPv6 routes (e.g. we might want static IPv4 routes only). Also, join the
"no" version of the command in the same DEFUN (Yes We Can).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>