RFR: 8284890: Support for Do not fragment IP socket options [v3]

[Michael McMahon]

> Hi,
> 
> Could I get the following PR review please? It adds a new JDK specific extended socket option
> called IP_DONTFRAGMENT, which disables IP packet fragmentation in both IPv4 and IPv6
> UDP sockets (NIO DatagramChannels). For IPv4 in particular, it sets the DF (Dont Fragment) bit
> in the IP header. There is no equivalent in the IPv6 packet header as fragmentation is implemented
> exclusively by the sending and receiving nodes.
> 
> Thanks,
> Michael

Michael McMahon has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 25 additional commits since the last revision:

 - windows update
 - test update
 - Merge branch 'master' into mtu
 - builds in github action now
 - fix whitespace errors
 - minor spec update
 - windows 2016 issue
 - windows issue
 - windows update
 - test update
 - ... and 15 more: https://git.openjdk.java.net/jdk/compare/3b3bec01...e90aa7c3

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8245/files
  - new: https://git.openjdk.java.net/jdk/pull/8245/files/14c776b1..e90aa7c3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8245&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8245&range=01-02

  Stats: 148852 lines in 1314 files changed: 108219 ins; 7158 del; 33475 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8245.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8245/head:pull/8245

PR: https://git.openjdk.java.net/jdk/pull/8245