java/openjdk*: Allow users to specify an alternative cacerts file

Similar to certificate management with certctl(8) users can use custom
enterprise CAs. These need to be assembled into a custom cacerts file.
Therefore, an admin can install a custom truststore to an arbitrary
location and have the OpenJDK port symlink to it by adding
OPENJDK_SYMLINK_CACERTS to make.conf.
E.g., OPENJDK_SYMLINK_CACERTS=/usr/local/etc/ssl/cacerts.

PR: 229329
Approved by: jrm (mentor), otis (mentor), glewis
Differential Revision: https://reviews.freebsd.org/D42720