Implement:
readonly -q VAR...
readonly -p VAR...
export -q [-x] VAR...
export -p [-x] VAR...
all available only in !SMALL shells - and while here, limit
"export -x" to full sized shells as well.
Also, do a better job of arg checking and validating of the
export and readonly commands (which is really just one built-in)
and of issuing error messages when something bogus is detected.
Since these commands are special builtin commands, any error
causes shell exit (for non-interactive shells).
UnifiedSplitRaw