Unset PWD if it is incorrect and no value for it can be determined.
This preserves the logical current directory across shell invocations.
Example (assuming /home is a symlink):
$ cd
$ pwd
/home/foo
$ sh
$ pwd
/home/foo
Formerly the second pwd would show the physical path (symlinks resolved).
Current versions pass this test trivially by never importing PWD, but I plan
to change sh to import PWD if it is an absolute pathname for the current
directory, possibly containing symlinks.
