support <:O>: if a label is not found, continue execution after the go-to statement

* this is a SciTECO extension - it's not in TECO-11
* Allows for select-case-like constructs with default-clauses as in
  :Os.^EQa$
    !* default *!
  !s.foo!
    !* ... *!
  !s.bar!
    !* ... *!
* Consistent with nOlabel0,label1,...$ if <n> is out of range.
  Unfortunately this form of computed goto is not applicable when
  "selecting" by strings or non-consecutive integers.
* In order to continue after the <:O> statement, we must keep the
  program counter along with the label we were looking for.
  At the end of the macro, the PC is restored instead of throwing
  an error.
* Since that would be very inefficient in loops - where potentially
  all iterations would result in rescanning till the end of the
  macro - we now store a completed-flag in the goto table.
  If it is set while trying to :O to an unknown label, we can
  just continue execution.

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