[Bug 3901] LIB_GETBUF isn't thread-safe.

[Bug 3901] LIB_GETBUF isn't thread-safe.
[Bug 3900] fast_xmit() selects wrong local addr responding to mcast on
           Windows.
[Bug 3888] ntpd with multiple same-subnet IPs using manycastclient creates
           duplicate associations.
Remove useless pointer to Windows Help from system error messages.
Avoid newlines within Windows error messages.
Ensure unique association IDs if wrapped.
Write frequency file to 6 digits after decimal.
Unpeer on shutdown as long done on POSIX systems.

Simplify calc_addr_distance().
Clamp min/maxpoll in edge cases in newpeer().
Quiet local addr change logginig when unpeering.
Correct missing arg for %s printf specifier in
  send_blocking_resp_internal().

bk: 65ae110eFPxXkBR_6G30Dthy7bfOzQ