symm/...: Reformat encryption mode loops and related code.

  * Rename the various variables consistently.  Now `off' is the progress
    into a buffer, `b' is the buffer or reservoir, `t' and `u' are
    temporary internal-format blocks, `y' is a temporary octet.

  * Hoist variable declarations to function top-levels.

  * Squish compound statement bodies vertically.

  * Invert some conditions to reduce nesting depth.

  * Move loop-variable updates closer to where the thing they measure is
    actually used.

  * Elide pointless use of `register' storage class.

  * Remove spaces around diadic `*' and `/' operators.