patch 8.1.1979: code for handling file names is spread out

Problem:    Code for handling file names is spread out.
Solution:   Move code to new filepath.c file.  Graduate FEAT_MODIFY_FNAME.
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 8050f32..6e0439e 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -738,6 +738,7 @@
 	$(OUTDIR)\ex_eval.obj \
 	$(OUTDIR)\ex_getln.obj \
 	$(OUTDIR)\fileio.obj \
+	$(OUTDIR)\filepath.obj \
 	$(OUTDIR)\findfile.obj \
 	$(OUTDIR)\fold.obj \
 	$(OUTDIR)\getchar.obj \
@@ -1501,6 +1502,8 @@
 
 $(OUTDIR)/fileio.obj:	$(OUTDIR) fileio.c  $(INCL)
 
+$(OUTDIR)/filepath.obj:	$(OUTDIR) filepath.c  $(INCL)
+
 $(OUTDIR)/findfile.obj:	$(OUTDIR) findfile.c  $(INCL)
 
 $(OUTDIR)/fold.obj:	$(OUTDIR) fold.c  $(INCL)
@@ -1781,6 +1784,7 @@
 	proto/ex_eval.pro \
 	proto/ex_getln.pro \
 	proto/fileio.pro \
+	proto/filepath.pro \
 	proto/findfile.pro \
 	proto/getchar.pro \
 	proto/hardcopy.pro \