[SPARK-2996] Implement userClassPathFirst for driver, yarn.

Yarn's config option `spark.yarn.user.classpath.first` does not work the same way as
`spark.files.userClassPathFirst`; Yarn's version is a lot more dangerous, in that it
modifies the system classpath, instead of restricting the changes to the user's class
loader. So this change implements the behavior of the latter for Yarn, and deprecates
the more dangerous choice.

To be able to achieve feature-parity, I also implemented the option for drivers (the existing
option only applies to executors). So now there are two options, each controlling whether
to apply userClassPathFirst to the driver or executors. The old option was deprecated, and
aliased to the new one (`spark.executor.userClassPathFirst`).

The existing "child-first" class loader also had to be fixed. It didn't handle resources, and it
was also doing some things that ended up causing JVM errors depending on how things
were being called.