RFR: JDK-8258479: Minor cleanups in VMError

[Thomas Stuefe]

A list of smaller cleanups, each one too small to justify an own issue:

1) VMError exposes is_error_reported(), fatal_error_in_progress() and get_first_error_tid(). These functions are used from outside to check on the state of error handling. They can be combined and duplicate coding removed. Also, VMError::_error_reported can be removed, since its redundant with VMError::_first_error_tid != -1)
2) A lot of methods are unnecessarily public
3) the `env_var` array in vmError.cpp can be limited to file scope
4) definition of VMError static members should be combined to the start of the cpp file
5) the comment describing VMError::report() is misplaced
6) VMError::reset_signal_handlers() is badly named; nothing is reset here, the secondary signal handler is installed.

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/1802/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1802&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258479
  Stats: 132 lines in 7 files changed: 45 ins; 62 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1802.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1802/head:pull/1802

PR: https://git.openjdk.java.net/jdk/pull/1802