Rename /usr/src/sys/machine to /usr/src/sys/platform. Give the platform

Rename /usr/src/sys/machine to /usr/src/sys/platform.  Give the platform
name its own variable, MACHINE_PLATFORM, instead of trying to use MACHINE
to name it.  Adjust the build infrastructure to match.

Revert MACHINE back to its original definition and remove uname shims.
This removes confusion with third party software.

This means a pc32 build has MACHINE=i386 and MACHINE_ARCH=i386, and a
vkernel build also has MACHINE=i386 and MACHINE_ARCH=i386.  The new
MACHINE_PLAFORM would be pc32 for a pc32 build, and vkernel for a vkernel
build.

Adjust all kernel configuration files to specify platform, machine, AND
machine_arch.