Propagate the OS assigned port value (fixes LocalPort 0)

[zeroSteiner]

Propagate the OS-assigned port value. This means when LocalPort is 0, you get the value that the OS assigned.

See this comment for context rapid7/metasploit-framework#21604 (comment)

[kx7m2qd]

core change in local.rb looks right, gating the localport update in the else branch means it only runs after a successful bind, and the specs check out for both tcp server and udp.

one thing I couldn't figure out though proxies.rb only has require 'uri' added, nothing else changed there. not obvious how that ties into propagating the os-assigned port, is it needed for something else in this PR or just something you noticed while working on this?

also since proxies.rb is touched, should there be a spec for LocalPort 0 going through a proxy too, or are the two direct-bind specs enough for what this PR is trying to fix?

asking because rapid7/metasploit-framework#21604 depends on this landing, wanted to actually understand what's changing before that work gets built on top of it