logind: change to -1+errno

logind: change to -1+errno

Set errno and return -1 in public API calls like all other weston code
does. Most systemd+dbus calls return negative error-codes instead of -1
and setting errno. Thus, we need to explicitly set errno before returning.

Also note that we must set errno _after_ the cleanup path. Calling
functions like close() in the cleanup path might overwrite errno (which is
not what we want). So protect errno until the final return -1;