libdfp: Rewrite fpclassify to use libdecnumber

This patch refactor fpclassifyd[32|64|128] implementation to use
libdecnumber and avoid internal DPD logic in default implementation.
It also adds more FP_ZERO tests.
libdecnumber: Cleanup symbol names

This patch removes the decimal[32|64|128]Symbol.h headers to cleanup
up the symbol names.  This removes the internal rename of symbol that
adds the '___' prefix (which is not really necessary for libdfp).
libcecnumber: decNumberSymbols.h name cleanup

This patch removes the decNumberSymbols.h header which redefines
libdecnumber internal symbols by preprending them with "___" string.