RFR: 8283337: Posix signal handler modification warning triggering incorrectly

[Kevin Walls]

PosixSignals::print_signal_handler() is incorrectly detecting a changed signal handler.  The signal handler for SIGBREAK/SIGQUIT is now set in src/hotspot/os/posix/signals_posix.cpp with the bool parameter do_check set to false.

set_signal_handler should only store a handler in vm_handlers when do_check is true.

However I don't see a simple way of getting a valid warning for the SIGQUIT handler, if it is added later when os::initialize_jdk_signal_support() calls os::signal().

If only signals added directly by src/hotspot/os/posix/signals_posix.cpp have the warning for a handler changing, then we never had a warning for SIGQUIT, so just this simple change is needed to remove the bogus warning.

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Commit messages:
 - 8283337: Posix signal handler modification warning triggering incorrectly

Changes: https://git.openjdk.java.net/jdk/pull/7858/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7858&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283337
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7858.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7858/head:pull/7858

PR: https://git.openjdk.java.net/jdk/pull/7858