Support legacy environment markers in PEP 508

[gsnedders]

packaging supports a few environment markers that are not currently in PEP 508:

• os.name, sys.platform, platform.version, platform.machine, and platform.python_implementation from PEP 345; and
• python_implementation from setuptools.

Given the goal of backwards compatibility, we probably need these redundant duplicates included. python_implementation especially is widely relied on.