patch 8.1.0673: functionality for signs is spread out over several files

Problem:    Functionality for signs is spread out over several files.
Solution:   Move most of the sign functionality into sign.c. (Yegappan
            Lakshmanan, closes #3751)
diff --git a/src/Make_bc5.mak b/src/Make_bc5.mak
index 6725c6d..a5ee13b 100644
--- a/src/Make_bc5.mak
+++ b/src/Make_bc5.mak
@@ -581,6 +581,7 @@
 	$(OBJDIR)\screen.obj \
 	$(OBJDIR)\search.obj \
 	$(OBJDIR)\sha256.obj \
+	$(OBJDIR)\sign.obj \
 	$(OBJDIR)\spell.obj \
 	$(OBJDIR)\spellfile.obj \
 	$(OBJDIR)\syntax.obj \