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/Makefile b/src/Makefile
index 6e676f1..cae0e5f 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1668,6 +1668,7 @@
 	version.c \
 	viminfo.c \
 	window.c \
+	bufwrite.c \
 	$(OS_EXTRA_SRC)
 
 SRC =	$(BASIC_SRC) \
@@ -1804,6 +1805,7 @@
 	objects/version.o \
 	objects/viminfo.o \
 	objects/window.o \
+	objects/bufwrite.o \
 	$(GUI_OBJ) \
 	$(TERM_OBJ) \
 	$(LUA_OBJ) \
@@ -1966,6 +1968,7 @@
 	version.pro \
 	viminfo.pro \
 	window.pro \
+	bufwrite.pro \
 	beval.pro \
 	gui_beval.pro \
 	netbeans.pro \
@@ -3432,6 +3435,9 @@
 objects/window.o: window.c
 	$(CCC) -o $@ window.c
 
+objects/bufwrite.o: bufwrite.c
+	$(CCC) -o $@ bufwrite.c
+
 objects/netbeans.o: netbeans.c
 	$(CCC) -o $@ netbeans.c
 
@@ -3949,6 +3955,10 @@
  auto/osdef.h ascii.h keymap.h term.h macros.h option.h beval.h \
  proto/gui_beval.pro structs.h regexp.h gui.h alloc.h ex_cmds.h spell.h \
  proto.h globals.h
+objects/bufwrite.o: bufwrite.c vim.h protodef.h auto/config.h feature.h os_unix.h \
+ auto/osdef.h ascii.h keymap.h term.h macros.h option.h beval.h \
+ proto/gui_beval.pro structs.h regexp.h gui.h alloc.h ex_cmds.h spell.h \
+ proto.h globals.h
 objects/gui.o: gui.c vim.h protodef.h auto/config.h feature.h os_unix.h \
  auto/osdef.h ascii.h keymap.h term.h macros.h option.h beval.h \
  proto/gui_beval.pro structs.h regexp.h gui.h alloc.h ex_cmds.h spell.h \