Move all JI setAccessible to trySetAccessible.

[headius]

The logic previously was depending on the ji.setAccessible
property, which by default would simply refuse to set accessible
at all when the Java version was >= "1.9" (also showing this
logic was introduced very early in the Java 9 development cycle.)

That default is removed here, along with a the CAN_SET_ACCESSIBLE
static field in RubyInstanceConfig deprecated and replaced with
the original meaning of this property, simply whether to attempt
setAccessible.

The direct calls to setAccessible have been replaced by the
trySetAccessible in backport9 library that checks if the target
member's module is open to JRuby, allowing users to opt-in to that
reflective access if their application needs it. Previously these
cases still skipped setAccessible just due to being on Java 9+.

Most of the changes here were masked by the property, but I have
also fixed several others that were not guarded by the property
or that were otherwise used in booting JRuby or extending Java
integration.

This fixes #5841 by allowing java_method Method objects to
go ahead and trySetAccessible when the module is open.

This relates to #5821, which fixed in 9.2.8 a similar issue where
even an open module would not set fields and methods accessible.

[kares]

great, we're finally behave 99.9+% same on 8 as on Java 9+

minor suggestion: have a JRuby trySetAccessible helper
... so we do not have to keep in mind to pass Ruby.class

[headius]

minor suggestion: have a JRuby trySetAccessible helper
... so we do not have to keep in mind to pass Ruby.class

Yeah that might be a good idea. I hate to add something more to Ruby.java, but maybe to our Java class? It's already pretty fat and not really public API. Java.trySetAccessible would just do Modules.trySetAccessible(..., Java.class) and have the same effect.