[foreign-memaccess] RFR: 8241011: Internal access modes should be filtered

[Maurizio Cimadamore]

This patch fixes some of the issues introduced by JDK-8240874; since MemorySegmentImpl is using some internal flags, we
should filter out internal flags when user queries for access modes. Moreover, we should detect situations where the
user is trying to set or set access modes which fall outside the supported mask.

Few tests have been added to catch these situations.

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Commit messages:
 - JDK-8241011: Internal access modes should be filtered

Changes: https://git.openjdk.java.net/panama-foreign/pull/52/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/52/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241011
  Stats: 30 lines in 2 files changed: 26 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/52.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/52/head:pull/52

PR: https://git.openjdk.java.net/panama-foreign/pull/52