If possible, replace symbolic links in place

Naive 'ln -s -f a b' will put the link inside 'b', if
'b' is (a symlink to) a directory; so, during upgrades,
the links in $(sysconfdir) cannot be updated this way.
Removing and readding works, however leaves the risk
of the upgrade process dying in that very moment, thus
leaving 'gnt-cluster' a dangling link, so that the --resume
option is of no help. On GNU systems, avoid this problem
by using the -T option of GNU ln.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

lib/client/gnt_cluster.py

@@ -1723,16 +1723,26 @@ def _SetGanetiVersion(versionstring):
 
   """
   failed = []
-  failed.extend(_VerifyCommand(
-      ["rm", "-f", os.path.join(pathutils.SYSCONFDIR, "ganeti/lib")]))
-  failed.extend(_VerifyCommand(
-      ["ln", "-s", "-f", os.path.join(pathutils.PKGLIBDIR, versionstring),
-       os.path.join(pathutils.SYSCONFDIR, "ganeti/lib")]))
-  failed.extend(_VerifyCommand(
-      ["rm", "-f", os.path.join(pathutils.SYSCONFDIR, "ganeti/share")]))
-  failed.extend(_VerifyCommand(
-      ["ln", "-s", "-f", os.path.join(pathutils.SHAREDIR, versionstring),
-       os.path.join(pathutils.SYSCONFDIR, "ganeti/share")]))
+  if constants.HAS_GNU_LN:
+    failed.extend(_VerifyCommand(
+        ["ln", "-s", "-f", "-T",
+         os.path.join(pathutils.PKGLIBDIR, versionstring),
+         os.path.join(pathutils.SYSCONFDIR, "ganeti/lib")]))
+    failed.extend(_VerifyCommand(
+        ["ln", "-s", "-f", "-T",
+         os.path.join(pathutils.SHAREDIR, versionstring),
+         os.path.join(pathutils.SYSCONFDIR, "ganeti/share")]))
+  else:
+    failed.extend(_VerifyCommand(
+        ["rm", "-f", os.path.join(pathutils.SYSCONFDIR, "ganeti/lib")]))
+    failed.extend(_VerifyCommand(
+        ["ln", "-s", "-f", os.path.join(pathutils.PKGLIBDIR, versionstring),
+         os.path.join(pathutils.SYSCONFDIR, "ganeti/lib")]))
+    failed.extend(_VerifyCommand(
+        ["rm", "-f", os.path.join(pathutils.SYSCONFDIR, "ganeti/share")]))
+    failed.extend(_VerifyCommand(
+        ["ln", "-s", "-f", os.path.join(pathutils.SHAREDIR, versionstring),
+         os.path.join(pathutils.SYSCONFDIR, "ganeti/share")]))
   return list(set(failed))