Simplify krb5_krcc_start_seq_get

This code can be simplified (and a potential race avoided) by using
keyctl_read_alloc() and letting it allocate the necessary memory.
This also allows to remove a helper function that is not used anymore
as well as make the code more readable.  The only penalty is that we
have two allocations instad of one.

[ghudson@mit.edu: trivial simplifications]