Clean the modification of ccstr JVM flags

There are two versions of JVMFlagAccess::ccstrAtPut() for modifying JVM flags of the ccstr type (i.e., strings).

See https://github.com/openjdk/jdk/blob/4e74de4b2eec611b49ee8defae1ab06351280008/src/hotspot/share/runtime/flags/jvmFlagAccess.cpp

- 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.