This change makes sure that, when we set the ruid and euid in order to
get the user keyring linked into the current process keyring, we will
always be able to reverse these changes (using a suid of 0).

This fixes an issue where "su <user>" would result in a system error
when called by an unprivileged user. It also explains exactly how and
why we are making these privilege changes.

Now instead of spawning a seperate thread we alternate between changing
the euid and ruid to both find the keyring and link it to the process
keyring. Note that we also ensure that the user keyring is linked into
the root keyring whenever possible.

The keyring lookup functions no longer read from /proc/keys. Now they
simply spawn a thread, drop privs, and check with GetKeyringID and
KEY_SPEC_USER_KEYRING. See userKeyringID() for more info.

The privileges functions have also been changed. Now the concept of
setting privileges is seperate form the concept of setting up the
keyrings.