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/README.md b/src/README.md
index 0b6a311..dcc7a5c 100644
--- a/src/README.md
+++ b/src/README.md
@@ -36,6 +36,7 @@
 evalfunc.c	| built-in functions
 evalvars.c	| vim variables
 fileio.c	| reading and writing files
+filepath.c	| dealing with file names and paths
 findfile.c	| search for files in 'path'
 fold.c		| folding
 getchar.c	| getting characters and key mapping