RFR: 8264285: Clean the modification of ccstr JVM flags [v3]

[Ioi Lam]

> There are two versions of JVMFlagAccess::ccstrAtPut() for modifying JVM flags of the ccstr type (i.e., strings).
> 
> - One version requires the caller to free the old value, but some callers don't do that (writeableFlags.cpp).
> - The other version frees the old value on behalf of the caller. However, this version is accessible only via FLAG_SET_XXX macros and is currently unused. So it's unclear whether it actually works.
> 
> We should combine these two versions into a single function, fix problems in the callers, and add test cases. The old value should be freed automatically, because typically the caller isn't interested in the old value.
> 
> Note that the FLAG_SET_XXX macros do not return the old value. Requiring the caller of FLAG_SET_XXX to free the old value would be tedious and error prone.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  Revert "relax flag attributions (ala JDK-7123237)"
  
  This reverts commit 673aaafc4860dd7f70a3f222422ae84e85fd4219.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3254/files
  - new: https://git.openjdk.java.net/jdk/pull/3254/files/673aaafc..2a5e2b19

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3254&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3254&range=01-02

  Stats: 37 lines in 4 files changed: 36 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3254.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3254/head:pull/3254

PR: https://git.openjdk.java.net/jdk/pull/3254