RFR: 8292695: SIGQUIT and jcmd attaching mechanism does not work with signal chaining library [v3]

[Man Cao]

> Hi all,
> 
> Could anyone review this bug fix? See https://bugs.openjdk.org/browse/JDK-8292695 for details.
> 
> I changed the temporary handler for SIGQUIT to use a dummy function, and use `os::signal()` to set it up, just as `os::initialize_jdk_signal_support()` does.
> It is possible that just moving the `set_signal_handler(BREAK_SIGNAL, false);` in `install_signal_handlers()` outside of the window bounded by `JVM_{begin|end}_signal_setting()` could also fix this bug. However, `set_signal_handler()` and `JVM_HANDLE_XXX_SIGNAL()` are currently used for signals that support chaining and periodically check, which do not apply to SIGQUIT. I think it is cleaner to use different functions for SIGQUIT.
> 
> I also added a test to check that sending SIGQUIT should produce a thread dump on stdout, with and without using libjsig.so.
> 
> -Man

Man Cao has updated the pull request incrementally with one additional commit since the last revision:

  Move jdk_misc_signal_init() earlier and clarify its comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9955/files
  - new: https://git.openjdk.org/jdk/pull/9955/files/d226de41..339d7eed

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9955&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9955&range=01-02

  Stats: 43 lines in 2 files changed: 22 ins; 17 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9955.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9955/head:pull/9955

PR: https://git.openjdk.org/jdk/pull/9955