Reset lexer state on syntax error and on SIGINT (Ctrl+C)

ksh crashed if you pressed Ctrl+C or Ctrl+D on a PS2 prompt while
you haven't finished entering a $(command substitution). It
corrupts subsequent command substitutions. Sometimes the situation
recovers, sometimes the shell crashes.

Simple crash reproducer:

  $ PS1="\$(echo foo) \$(echo bar) \$(echo baz) > "
  foo bar baz > echo $(                        <-- now press Ctrl+D
  > ksh: syntax error: `(' unmatched
  Memory fault

The same happens with Ctrl+C, minus the syntax error message.

The problem is that the lexer state becomes inconsistent when the
lexer is interrupted in the middle of reading a command
substitution of the form $( ... ). This is tracked in the
'lexd.dolparen' variable in the lexer state struct.

Resetting that variable is sufficient to fix this issue. However,
in this commit I prefer to just reinitialise the lexer state
completely to pre-empt any other possible issues. Whether there was
a syntax error or the user pressed Ctrl+C, we just interrupted all
lexing and parsing, so the lexer *should* restart from scratch.

src/cmd/ksh93/sh/fault.c: sh_fault():
- If the shell is in an interactive state (e.g. not a subshell) and
  SIGINT was received, reinitialise the lexer state. This fixes the
  crash with Ctrl+C.

src/cmd/ksh93/sh/lex.c: sh_syntax():
- When handling a syntax error, reset the lexer state. This fixes
  the crash with Ctrl+D.

NEWS:
- Also add the forgotten item for the previous fix (re: 2322f939).

src/cmd/ksh93/sh/fault.c

@@ -68,6 +68,9 @@ void	sh_fault(register int sig)
 	register char		*trap;
 	register struct checkpt	*pp = (struct checkpt*)shp->jmplist;
 	int	action=0;
+	/* reset lexer state on Ctrl+C */
+	if(sh_isstate(SH_INTERACTIVE) && sig==SIGINT)
+		sh_lexopen(sh.lex_context, &sh, 0);
 	/* reset handler */
 	if(!(sig&SH_TRAP))
 		signal(sig, sh_fault);