Improve data validation:

Improve data validation:
- Simplify the check for a well-formed -a argument.
- Move the check for overflowing the command table from addcmd()
  (which is just a driver) into addmac() (which does the real work,
  and which is called from places besides addcmd().)
- Add a check for overflowing the macro-pair table.
- In each case, use a clearer (while) loop for detecting overflow.
- In each case, error out by using errx().