tr(1): rename lookup tables

tr(1): rename lookup tables

The lookup tables in tr(1) are called "string1" and "string2".  This is
misleading:

- They aren't C strings, they are lookup tables.

- The names "string1" and "string2" don't hint at what their contents
  actually mean.  The meaning of a given table changes with tr(1)'s
  operating mode.  There are five different modes.

It would be considerably easier to see what is happening at a glance
if the tables were named for their corresponding byte transformation.

So instead of two tables named "string1" and "string2" we'll have
three tables named "delete", "squeeze", and "translate".  In addition,
the "string" passed to the setup() function will be called a "table".

With this patch the code in main() is way easier to understand.
Hopefully this makes subsequent patches easier to review.

Thread: https://marc.info/?l=openbsd-tech&m=164355980600901&w=2

No complaints on tech@ after over a week.