Support OCaml 4.06

• add 9pmount-vsock and tap-vsock helper programs
• add missing command-line options and support running without the database
• add go library and helper tools to expose ports
• tcp: enable keep-alives
• tcp: disable nagle
• udp: drop packets with incorrect source addresses
• test: record one .pcap trace per test
• icmp: add support for ping
• dns: use persistent TCP connections but transient UDP "connections" to increase
  the request entropy
• dns: increase scalability on the Mac
• http: add a regular HTTP proxy (as well as the transparent one)
• windows: use RtlGenRandom for entropy
• windows: be more robust to Hyper-V socket failures
• fix build with -safe-string and OCaml 4.06
• support builds with the system OCaml compiler
• socket protocol updated to v22:
  • support error messages returned to client for Ethernet and Preferred_ipv4
    slirp commands
  • allow client to request an IPv4 address without encoding it in the UUID
  • v1 no longer supported, clients have to be updated. Version 22 is used to
    match the current version number in Docker for Desktop.