progs/perftest.c: Fix key-size handling.

  * Allow a key-size parameter to `enc', because algorithms like
    Rijndael have key-size-dependent performance.  This uses the `-b'
    option, because `-B' is already the buffer size for the inner loop.

  * For consistency, use `-b' for the key size in `ksched' too.

  * Finally, check explicit key sizes for validity rather than just
    rounding off and potentially crashing.