Restore default argument and nargs on bind-addr

This is required to allow bind-addr to specify only an IPv6 address.

The goal is to set the default as 'localhost' if the user doesn't specify an address, but if you use bind-addr, you are expected to provide all applicable addresses.

Example use of an IPv6 only sandbox:

pgbox init --pgbin /usr/lib/postgresql/10/bin \\
    --port=12345 --box ./box ---bind-addr ip6-localhost \\
--  -E UTF8 --locale=C

Signed-off-by: Robert Roland robert.roland@puppet.com