stat-util: rename files_same() → inode_same()

Let's be more accurate about what this function does: it checks whether
the underlying reported inode is the same. Internally, this already uses
a better named stat_inode_same() call, hence let's similarly name the
wrapping function following the same logic.

Similar for files_same_at() and path_equal_or_same_files().

No code changes, just some renaming.

src/basic/stat-util.c

@@ -183,7 +183,7 @@ int path_is_read_only_fs(const char *path) {
         return fd_is_read_only_fs(fd);
 }
 
-int files_same_at(int fda, const char *filea, int fdb, const char *fileb, int flags) {
+int inode_same_at(int fda, const char *filea, int fdb, const char *fileb, int flags) {
         struct stat a, b;
 
         assert(fda >= 0 || fda == AT_FDCWD);