chase-symlinks: Rename chase_symlinks() to chase()

Chasing symlinks is a core function that's used in a lot of places
so it deservers a less verbose names so let's rename it to chase()
and chaseat().

We also slightly change the pattern used for the chaseat() helpers
so we get chase_and_openat() and similar.

src/basic/fileio.c

@@ -13,7 +13,7 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
-#include "chase-symlinks.h"
+#include "chase.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "fs-util.h"