Merge JVM and OS trust stores in case opt-in system property

  eclipse.platform.mergeTrust

is set and trust is not otherwise customized via the

  javax.net.ssl.trustStore[...]

system properties.

Resolves:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=567504
https://github.com/eclipse-platform/eclipse.platform/issues/1690

Obsoletes (to-be-reverted):
https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/pull/929
https://github.com/eclipse-packaging/packages/pull/224

Replaces:
https://github.com/eclipse-equinox/equinox/pull/1176

See also:
https://github.com/eclipse-simrel/.github/pull/38