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_morph.mak b/src/Make_morph.mak
index 432efa2..09e514c 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -49,6 +49,7 @@
 	ex_eval.c						\
 	ex_getln.c						\
 	fileio.c						\
+	filepath.c						\
 	findfile.c						\
 	fold.c							\
 	getchar.c						\