RFR: 7903347: add long name option for all single letter options and expand help on default values for various options

[Athijegannathan Sundararajan]

* long option names for -I, -l, -D (--include-dir, --library, --define-macro)
* documented default values for header class name, target package and output directory
* explanation for -l / --library option.

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Commit messages:
 - 7903347: add long name option for all single letter options and expand help on default values for various options
 - fixed help message for -l/--library. Sync'ed descriptive text in README.md table with --help output.
 - long options for -I, -l, -D. document default values used for header class name, target package, output dir,

Changes: https://git.openjdk.org/jextract/pull/83/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=83&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903347
  Stats: 145 lines in 10 files changed: 102 ins; 6 del; 37 mod
  Patch: https://git.openjdk.org/jextract/pull/83.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/83/head:pull/83

PR: https://git.openjdk.org/jextract/pull/83