patch 8.1.1766: code for writing session file is spread out

Problem:    Code for writing session file is spread out.
Solution:   Put it in one file. (Yegappan Lakshmanan, closes #4728)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 10fcb74..04cdd08 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -767,6 +767,7 @@
 	$(OUTDIR)\regexp.obj \
 	$(OUTDIR)\screen.obj \
 	$(OUTDIR)\search.obj \
+	$(OUTDIR)\session.obj \
 	$(OUTDIR)\sha256.obj \
 	$(OUTDIR)\sign.obj \
 	$(OUTDIR)\spell.obj \
@@ -1618,6 +1619,8 @@
 
 $(OUTDIR)/search.obj:	$(OUTDIR) search.c  $(INCL)
 
+$(OUTDIR)/session.obj:	$(OUTDIR) session.c  $(INCL)
+
 $(OUTDIR)/sha256.obj:	$(OUTDIR) sha256.c  $(INCL)
 
 $(OUTDIR)/sign.obj:	$(OUTDIR) sign.c  $(INCL)
@@ -1788,6 +1791,7 @@
 	proto/regexp.pro \
 	proto/screen.pro \
 	proto/search.pro \
+	proto/session.pro \
 	proto/sha256.pro \
 	proto/sign.pro \
 	proto/spell.pro \