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_sas.mak b/src/Make_sas.mak
index d07fb55..a16908e 100644
--- a/src/Make_sas.mak
+++ b/src/Make_sas.mak
@@ -134,6 +134,7 @@
 	screen.c \
 	search.c \
 	sha256.c \
+	sign.c \
 	spell.c \
 	spellfile.c \
 	syntax.c \
@@ -191,6 +192,7 @@
 	screen.o \
 	search.o \
 	sha256.o \
+	sign.o \
 	spell.o \
 	spellfile.o \
 	syntax.o \
@@ -248,6 +250,7 @@
 	proto/screen.pro \
 	proto/search.pro \
 	proto/sha256.pro \
+	proto/sign.pro \
 	proto/spell.pro \
 	proto/spellfile.pro \
 	proto/syntax.pro \
@@ -404,6 +407,8 @@
 proto/search.pro:	search.c
 sha256.o:		sha256.c
 proto/sha256.pro:	sha256.c
+sign.o:			sign.c
+proto/sign.pro:		sign.c
 spell.o:		spell.c
 proto/spell.pro:	spell.c
 spellfile.o:		spellfile.c