Use realpath by default to resolve symlinks instead of readlink -f

Older versions of busybox do not support the -f flag for readlink. realpath on
the other hand works both for the GNU coreutils & busybox.

To be safe, we still fall back to readlink -f in case realpath is not available
on the system.

Author: dcermak

testinfra/modules/file.py

@@ -61,6 +61,9 @@ def linked_to(self):
         >>> host.file("/var/lock").linked_to
         '/run/lock'
         """
+        res = self.run_expect([0, 127], "realpath %s", self.path)
+        if res.rc == 0:
+            return res.stdout.strip()
         return self.check_output("readlink -f %s", self.path)
 
     @property