RFR: 7903347: add long name option for all single letter options and expand help on default values for various options [v2]

[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.

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  Update README.md
  
  Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jextract/pull/83/files
  - new: https://git.openjdk.org/jextract/pull/83/files/560ec003..7a66dc99

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=83&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=83&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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