RFR: 8329728: Read arbitrarily long lines in ClassListParser

[Ioi Lam]

Today the `ClassListParser` has a hard-coded limit of 4096 chars for each line in the CDS class list file. However, it's possible for a line to be much longer than than (64KB for the class name, plus extra information that can include path names, IDs, etc).

I wrote a utility class `LineReader` that automatically allocates a buffer before calling `fgets()`. Hopefully this can be useful for other cases where we call `fgets()` with a fixed buffer size.

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Commit messages:
 - 8329728: Read arbitrarily long lines in ClassListParser

Changes: https://git.openjdk.org/jdk/pull/18669/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18669&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329728
  Stats: 235 lines in 5 files changed: 193 ins; 18 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/18669.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18669/head:pull/18669

PR: https://git.openjdk.org/jdk/pull/18669