Further improve handling of symlink targets/deps.

In commit redo-0.11-4-g34669fb, we changed os.stat into os.lstat to
avoid false positives in the "manual override" detector: a .do file
that generates $3 as a symlink would trigger manual override if the
*target* of that symlink ever changed, which is incorrect.

Unfortunately using os.lstat() leads to a different problem: if X
depends on Y and Y is a symlink to Z, then X would not be rebuilt when
Z changes, which is clearly wrong.

The fix is twofold:

1. read_stamp() should change on changes to both the link itself,
   *and* the target of the link.

2. We shouldn't mark a target as overridden under so many situations.
   We'll use *only* the primary mtime of the os.lstat(), not all the
   other bits in the stamp.

Step 2 fixes a few other false positives also.  For example, if you
'cp -a' a whole tree to another location, the st_ino of all the targets
will change, which would trigger a mass of "manual override" warnings.
Although a change in inode is sufficient to count an input as having
changed (just to be extra safe), it should *not* be considered a manual
override.  Now we can distinguish between the two.

Because the stamp format has changed, update the SCHEMA_VER field.  I
should have done this every other time I changed the stamp format, but
I forgot.  Sorry.  That leads to spurious "manually modified" warnings
after upgrading redo.

redo-ifchange.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python2
-import sys, os, traceback
+import os, sys, traceback
 
 import vars_init
 vars_init.init(sys.argv[1:])