This commit introduces the ability to override JVM options by adding
custom JVM options files to a jvm.options.d directory. This simplifies
administration of Elasticsearch by not requiring administrators to keep
the root jvm.options file in sync with changes that we make to the root
jvm.options file. Instead, they are not expected to modify this file but
instead supply their own in jvm.options.d. In Docker installations, this
means they can bind mount this directory in. In future versions of
Elasticsearch, we can consider removing the root jvm.options file
(instead, providing all options there as system JVM options).
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