IP allow lists for sshmux and fixes to proxy protocol for sshmux

Changelog for 0.9.37:

• Enables an allow list for the sshmux command's config file for IP allow lists - using CIDR or plain IP format, run with --help for an example of how to use this
• Fixes reliability for handling of proxy protocol for retrieving client IPs

If your home IP was 35.202.222.154, you could write:

upstreams:
  - name: nuc.example.com
    upstream: 192.168.0.120:22
    allowed:
    - 35.202.222.154

Changes: 0.9.36...0.9.37

Generated by Derek