8351740: Clean up some code around initialization of encoding properties

[stuart-marks]

Some of the code that creates various encoding properties at JVM initialization time, such as file.encoding and native.encoding, could use some cleaning up. Cleanup is fairly minimal and should be mostly behavior-preserving. Changes include the following:

• In the java_props.h and java_props_md.c files, add documentation and asserts that the sprops.encoding and sprops.sun_jnu_encoding members are always set to non-NULL by the platform-specific code.

• In SystemProps.java, remove a null check that is now extraneous based on the above assertion.

• In SystemProps.java, rearrange logic (with no behavior change) around handling of the "COMPAT" value for the file.encoding property.

• In SystemProps.Raw, rename the platform properties array index from _file_encoding_NDX to _native_encoding_NDX.

• In SystemProps.Raw.cmdProperties(), adjust the HashMap size computation. This should actually avoid resizing of the HashMap (unlike the earlier code).

• Adjust various comments in several places for clarity and correctness.

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Issue

• JDK-8351740: Clean up some code around initialization of encoding properties (Enhancement - P4)

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