RFR: 8293842: IPv6-only systems throws UnsupportedOperationException for several socket/TCP options

[Man Cao]

Hi all,

Could anyone review this bug fix for ipv6-only system? See https://bugs.openjdk.org/browse/JDK-8293842 for detailed description of this bug.

Ideally, the `socket(PF_INET6, ...)` or `socket(PF_INET, ...)` call should depend on the value of `ipv6_available()`. However, this is only easy to do in net_util_md.c. Without checking `ipv6_available()` in <Linux|MacOSX>SocketOptions.c, I'm not sure if it is possible for ipv6 and ipv4 sockets to differ in the options they support.

For example, for a system with both ipv6 and ipv4, if ipv6 supports TCP_KEEPIDLE but ipv4 does not, then there might be a problem to always report TCP_KEEPIDLE as supported. 

I noticed https://bugs.openjdk.org/browse/JDK-8290349 added "isIPv6" parameter to a few functions in <Linux|MacOSX>SocketOptions.c. However, it is not applicable to the `socketOptionSupported()` function. This change is more similar to the `Java_jdk_net_MacOSXSocketOptions_ipDontFragmentSupported0()` function.

-Man

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Commit messages:
 - 8293842: IPv6-only systems throws UnsupportedOperationException for several socket/TCP options

Changes: https://git.openjdk.org/jdk/pull/10278/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10278&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293842
  Stats: 25 lines in 4 files changed: 19 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10278.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10278/head:pull/10278

PR: https://git.openjdk.org/jdk/pull/10278