fixed bug where we looked at the first byte of a password to determine

fixed bug where we looked at the first byte of a password to determine
if the password is blank. That ain't valid with encrypted passwords!

Jeremy, this changes the semantics of session setup. We no longer
automatically set guest=True when the client happens to choose their
username == the guest username. Instead we rely on the map_to_guest
code. I'm pretty sure this is now the right thing, but please look at
it carefully before putting it in 2.2

This is the bug that was causing the build farm to sometimes
fail. Basically we failed every 256 attempts - ie. when the first byte
of the encrypted password happened to be 0
(This used to be commit 4b35a3494c53fe38532fc12cf1c57e1a3715ad0f)