preproc: classify warnings, move into common pp-* namespace

Classify all remaining WARN_OTHER warnings in the preprocessor. Move
all preprocessor warnings except "user" under a common pp-* prefix.

Warn for an out-of-range argument to the %sel() function.

Finally, use "dname" in additional places for consistency and future
ease of use.

Signed-off-by: H. Peter Anvin <[email protected]>

Author: H. Peter Anvin