DragonFlyBSD/src d000ce6lib/libm Makefile, lib/libm/man signbit.3

Add back a signbit.3 manpage that was killed w/o providing a substitute.
DeltaFile
+57-0lib/libm/man/signbit.3
+5-0lib/libm/Makefile
+62-02 files

UnifiedSplitRaw