Properly implement 'break' and 'continue' in C-style for loop.

Addresses issue #66.

core/cmd_exec.py

@@ -964,7 +964,6 @@ def _Dispatch(self, node, fork_external):
              break
            elif e.IsContinue():
              status = 0
              continue
            else:  # return needs to pop up more
              raise
      finally:
@@ -973,13 +972,30 @@ def _Dispatch(self, node, fork_external):
    elif node.tag == command_e.ForExpr:
      status = 0
      self.arith_ev.Eval(node.init)
      while True:
        b = self.arith_ev.Eval(node.cond)
        if not b:
          break

        status = self._Execute(node.body)
        self.arith_ev.Eval(node.update)
      self.loop_level += 1
      try:
        while True:
          b = self.arith_ev.Eval(node.cond)
          if not b:
            break

          do_continue = False
          try:
            status = self._Execute(node.body)
          except _ControlFlow as e:
            if e.IsBreak():
              status = 0
              break
            elif e.IsContinue():
              status = 0
            else:  # return needs to pop up more
              raise

          self.arith_ev.Eval(node.update)

      finally:
        self.loop_level -= 1

    elif node.tag == command_e.DoGroup:
      status = self._ExecuteList(node.children)

spec/for-expr.test.sh

@@ -4,16 +4,21 @@
# mksh implements most too.

### C-style for loop
n=5
n=10
for ((a=1; a <= n ; a++))  # Double parentheses, and naked 'n'
do
  if test $a = 3; then
    continue
  fi
  if test $a = 6; then
    break
  fi
  echo $a
done  # A construct borrowed from ksh93.
## status: 0
## STDOUT:
1
2
3
4
5
## N-I mksh status: 1