Add -4 and -6 options to flashproxy-client.

These are passed down to registration helpers that support them.
Add -4 -6 to flashproxy-client man page.
ChangeLog for -4 -6 in flashproxy-client.
Make separate reg pools for IPv4 and IPv6.

This is only cosmetic at the moment, as all regs will go into the IPv4
pool.
Add abstractions for using IPv4/IPv6 reg pools.

Still always going to the IPv4 pool.
Add a simple function to guess an address family.
Completing get_reg_for_proxy and put_reg in facilitator
ChangeLog for #6124.