patch 8.1.2094: the fileio.c file is too big

Problem:    The fileio.c file is too big.
Solution:   Move buf_write() to bufwrite.c. (Yegappan Lakshmanan,
            closes #4990)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 762b349..e853ae9 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -716,6 +716,7 @@
 	$(OUTDIR)\blob.obj \
 	$(OUTDIR)\blowfish.obj \
 	$(OUTDIR)\buffer.obj \
+	$(OUTDIR)\bufwrite.obj \
 	$(OUTDIR)\change.obj \
 	$(OUTDIR)\charset.obj \
 	$(OUTDIR)\cmdexpand.obj \
@@ -1457,6 +1458,8 @@
 
 $(OUTDIR)/buffer.obj:	$(OUTDIR) buffer.c  $(INCL)
 
+$(OUTDIR)/bufwrite.obj:	$(OUTDIR) bufwrite.c  $(INCL)
+
 $(OUTDIR)/change.obj:	$(OUTDIR) change.c  $(INCL)
 
 $(OUTDIR)/charset.obj:	$(OUTDIR) charset.c  $(INCL)
@@ -1788,6 +1791,7 @@
 	proto/blob.pro \
 	proto/blowfish.pro \
 	proto/buffer.pro \
+	proto/bufwrite.pro \
 	proto/change.pro \
 	proto/charset.pro \
 	proto/cmdexpand.pro \