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_manx.mak b/src/Make_manx.mak
index 6995b76..04560d4 100644
--- a/src/Make_manx.mak
+++ b/src/Make_manx.mak
@@ -81,6 +81,7 @@
 	screen.c \
 	search.c \
 	sha256.c \
+	sign.c \
 	spell.c \
 	spellfile.c \
 	syntax.c \
@@ -139,6 +140,7 @@
 	obj/screen.o \
 	obj/search.o \
 	obj/sha256.o \
+	obj/sign.o \
 	obj/spell.o \
 	obj/spellfile.o \
 	obj/syntax.o \
@@ -195,6 +197,7 @@
 	proto/screen.pro \
 	proto/search.pro \
 	proto/sha256.pro \
+	proto/sign.pro \
 	proto/spell.pro \
 	proto/spellfile.pro \
 	proto/syntax.pro \
@@ -389,6 +392,9 @@
 obj/sha256.o:	sha256.c
 	$(CCSYM) $@ sha256.c
 
+obj/sign.o:	sign.c
+	$(CCSYM) $@ sign.c
+
 obj/spell.o:	spell.c
 	$(CCSYM) $@ spell.c