Implement a cross machine login so that user is automatically logged

into the Wiki when clicking on the My Wiki's link. Works like this:

* The My Wiki's link points to new page, gotowiki.php3, on boss.

* The gotowiki page looks for a new cookie in the user's browser
  which holds a key (the usual random data run through md5).

* If the key does not exist, generate it and store it in the user
  browser (expires when browser is closed or emulab login times out).
  Also invoke backend script wikixlogin, which will send the key over
  to the wiki server (via ssh), which will write the key into a file
  named by the user account.

* The user's browser is redirected to the wiki server's login script
  (twiki/bin/newlogon), but instead of username and password, we send
  over username and key (as well as redurl= parameter which is the
  page on the wiki server to redirect to later).

* The new login script looks for this case, and opens the file named
  by the user and compares the key it gets with what is in the file.
  If they match, the user login succeeds and the browser is once again
  redirected, but this time to the page it wants on the wiki server.
  If the key does not match, the browser is redirected to the login
  page (so user can enter username password normally). The redurl
  parameter is passed along as well.

* Subsequent clicks on My Wiki's will not need to invoke the backend
  script, since the cookie will be in the browser.