lei: pass FD to CWD via cmsg, use fchdir on server

lei: pass FD to CWD via cmsg, use fchdir on server

Perl chdir() automatically does fchdir(2) if given a file
or directory handle since 5.8.8/5.10.0, so we can safely
rely on it given our 5.10.1+ requirement.

This means we no longer have to waste several milliseconds
loading the Cwd.so and making stat() calls to ensure
ENV{PWD} is correct and usable in the server.  It also lets
us work in directories that are no longer accessible via
pathname.