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/version.c b/src/version.c
index 3e06f92..a2b6ed2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -362,11 +362,7 @@
 #else
 	"-mksession",
 #endif
-#ifdef FEAT_MODIFY_FNAME
 	"+modify_fname",
-#else
-	"-modify_fname",
-#endif
 #ifdef FEAT_MOUSE
 	"+mouse",
 #  ifdef FEAT_MOUSESHAPE
@@ -762,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1979,
+/**/
     1978,
 /**/
     1977,