Fix regression caused by ${var:-'{}'} fix (re: f31e368)

The regression is:

  quoting.sh[189]: expansion of "{q:+'}" not correct when q unset

The failure was that, for unset q, "${q:+'}q${q:+'}" yielded empty
and not 'q'. This is because the single quotes within the double
quotes were erroneously parsed as meaningful.

The originally used ST_QUOTE state table (see data/lexstates.c),
where no quote character has any special meaning, was for avoiding
this problem.

The newly introduced ST_MOD1 state table is a copy of ST_QUOTE
except the ' has been given its special meaning back. We need this
to fix #290, but only for unquoted expansions.

So we need to go back to using ST_QUOTE if the string is quoted
(mp->quote) and we're not parsing a substitution that uses patterns
where quotes are significant (newops, ST_MOD2), i.e., only for
old-style ST_MOD1 operators.

src/cmd/ksh93/sh/macro.c: varsub():
- When the ${var<OP>string} expansion is quoted, and of an old
  (S_MOD1) type, then use the ST_QUOTE state table to skip over it
  instead of the new ST_MOD1 one.

src/cmd/ksh93/sh/macro.c

@@ -1659,7 +1659,12 @@ static int varsub(Mac_t *mp)
 			}
 			else
 			{
-				sh_lexskip(lp, RBRACE, 0, sh_lexstates[ST_BRACE][c]==S_MOD1 ? ST_MOD1 : ST_NESTED);
+				int state;
+				if(sh_lexstates[ST_BRACE][c]==S_MOD1)
+					state = mp->quote ? ST_QUOTE : ST_MOD1;
+				else
+					state = ST_NESTED;
+				sh_lexskip(lp, RBRACE, 0, state);
 				stkseek(stkp,offset);
 			}
 			argp=stkptr(stkp,offset);