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/proto.h b/src/proto.h
index cc31726..0eafcc4 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -84,6 +84,7 @@
 # include "ex_eval.pro"
 # include "ex_getln.pro"
 # include "fileio.pro"
+# include "filepath.pro"
 # include "findfile.pro"
 # include "fold.pro"
 # include "getchar.pro"