README: improve troubleshooting tips for unlocked encrypted files

README: improve troubleshooting tips for unlocked encrypted files

Rename the troubleshooting section "Can't log in with ssh even when
user's encrypted home directory is unlocked" to the more general "Some
processes can't access unlocked encrypted files", and rewrite it to
provide clearer directions for how to fix the problem by upgrading
encrypted directories to policy version 2.

Also add a related section "Users can access other users' unlocked
encrypted files" which covers the reverse "issue", i.e. people expecting
some processes to *not* be able to access unlocked encrypted files.

Fixes https://github.com/google/fscrypt/issues/248