Identify and eliminate dead or useless feature macros

[rdmark]

While we've made a lot of progress cleaning up obsolete code in recent years, I think there's still some residual platform/feature compatibility code floating around.

One I spotted today is the NO_REAL_USER_NAME macro in the UAM interface. To my best knowledge not a standard C macro, and not recognized by the build system. It originates from the 1.4.x era codebase (late 90s) and hasn't been touched since.