RFR: 8340380: Improve source launcher's shebang script detection [v3]

[Christian Stein]

> Please review this change that improves the launcher mode detection by reading the initial two characters from the started file for being a shebang script. It addresses the reported confusing error messages and also supports more shebang line variations. Including those line variations that omit the `--source` arguments like shown in the underlying issue description of JDK-8340380.
> 
> Tests of tier 1..3: _in progress_

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Flip initial checks to restore original behaviour
  
  Avoid regression encoded in `test/jdk/tools/launcher/SourceMode.testClassNamedJava()` assertions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21910/files
  - new: https://git.openjdk.org/jdk/pull/21910/files/b4b8a8d7..f0bd01d3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21910&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21910&range=01-02

  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21910.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21910/head:pull/21910

PR: https://git.openjdk.org/jdk/pull/21910