RFR: 8253742: POSIX signal code cleanup [v6]

[Gerard Ziemski]

> hi all,
> 
> Please review this followup to [JDK-8252324 Signal related code should be shared among POSIX platforms](https://bugs.openjdk.java.net/browse/JDK-8252324), where several issues were identified for a cleanup. This change addresses them all:
> 
> #1 David's feedback - removed non POSIX SIGNIFICANT_SIGNAL_MASK code
> 
> #2 David's feedback - used unblock_program_error_signals() on all platforms (reverted for JDK-8252533)
> 
> #3 David's feedback - used single JVM_handle_posix_signal API for all POSIX platforms (reverted for JDK-8255711)
> 
> #4 Coleen's feedback - cleanup header files in src/hotspot/os/posix/signals_posix.hpp
> 
> #5 Coleen's feedback - hid SR_signum assignment in src/hotspot/os/posix/signals_posix.hpp to avoid having to include <signal.h>
> 
> #6 Coleen's feedback - factored out print_signal_handlers()
> 
> #7 Thomas' feedback - factored out common POSIX os::SuspendedThreadTask::internal_do_task()
> 
> #8 Thomas's feedback - factored out common POSIX signal initialization code
> 
> #9 YaSuenag's feedback - used JVM_handle_posix_signal for the common API
> 
> #10 YaSuenag's feedback - unified logging out of the scope for this fix
> 
> #11 YaSuenag's feedback - memset usage in PosixSignals::jdk_misc_signal_init() correct?

Gerard Ziemski has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:

 - Merge master
 - Thomas' feedback - cleanup ucontext_get_pc/ucontext_set_pc
 - David's feedback
 - use ifdef(SIGDANGER) and ifdef(SIGTRAP)
 - revert unblock_program_error_signals change
 - revert JVM_handle_XXX_signal change
 - Factor out common POSIX signal initialization code
 - Factor out do_task into PosixSignals
 - factor out print_signal_handlers
 - Coleen's feedback integrated
 - ... and 6 more: https://git.openjdk.java.net/jdk/compare/17f04fc9...dbec19cc

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

Changes: https://git.openjdk.java.net/jdk/pull/636/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=636&range=05
  Stats: 331 lines in 21 files changed: 66 ins; 170 del; 95 mod
  Patch: https://git.openjdk.java.net/jdk/pull/636.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/636/head:pull/636

PR: https://git.openjdk.java.net/jdk/pull/636