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_cyg_ming.mak b/src/Make_cyg_ming.mak
index e362815..90a3461 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -729,6 +729,7 @@
 	$(OUTDIR)/ex_eval.o \
 	$(OUTDIR)/ex_getln.o \
 	$(OUTDIR)/fileio.o \
+	$(OUTDIR)/filepath.o \
 	$(OUTDIR)/findfile.o \
 	$(OUTDIR)/fold.o \
 	$(OUTDIR)/getchar.o \