WL#2067 add features to mysqltest: "disable_error_abort" + "$mysql_errno"

$mysql_errno is a new builtin variable of mysqltest and contains the
return code of the last command send to the server.

"--disable_abort_on_error" switches the abort of mysqltest
after "unmasked" failing statements off.
"--enable_abort_on_error" switches the abort of mysqltest
after "unmasked" failing statements on. (default)

"Maskings" are
    !$<error number>  and  --error <error number>
in the line before the statement to be checked.

The benefit of the option "--disable_abort_on_error" is that
- all statements after the failing statement are executed
- a r/<test>.reject will be produced
- it is possible to write test cases, which perform
  code sequences depending on the return code of a single
  statement