BUG #13793: Please implement IP_FREEBIND option

The following bug has been logged on the website:

Bug reference: 13793
Logged by: Tomasz Torcz
Email address: tomek(at)pipebreaker(dot)pl
PostgreSQL version: 9.4.5
Operating system: Linux
Description:

IP_FREEBIND socket option allows application to bind on IP address, which i
s not (yet) configured on any interface.
I'd like to see Postgresql binding to all specified 'listen_addresses', even
if some of then are not available at the moment pgsql starts.
Optionally, postmaster could put in logs information about that action.
Right know it is:

LOG: could not bind IPv6 socket: Cannot assign requested address
WARNING: could not create listen socket for "2001:DB8::1"

It could be:
LOG: could not bind IPv6 socket: Cannot assign requested address
WARNING: binding anyway to "2001:DB8::1"