Fix single quotes in expansion operator string (re: 5ed9ffd6)

The referenced commit introduced the following bug:

> The closing quote does not appear to be registering during the
> parse of the following:
>
>	echo ${var:+'{}'}
>
> Within a script, this will result in:
>
>	syntax error at line 1: `'' unmatched

src/cmd/ksh93/data/lexstates.c,
src/cmd/ksh93/include/lexstates.h:
- Add new ST_MOD1 state table that is a copy of ST_QUOTE, but adds
  a special meaning (ST_LIT) for the single quote (position 39).

src/cmd/ksh93/sh/lex.c: sh_lex():
- For parameter expansion operators with old-style quoting
  (S_MOD1), use the new ST_MOD1 state table instead of ST_QUOTE.
  This causes single quotes within them to be processed properly.

src/cmd/ksh93/tests/quoting2.sh:
- Add tests.

Thanks to @gkamat for the bug report.
Resolves: https://github.com/ksh93/ksh/issues/290

src/cmd/ksh93/tests/quoting2.sh

@@ -250,5 +250,18 @@ actual=$(printf %q $'1\x[11]1')
 [[ $actual == "$expect" ]] || err_exit 'shell-quoting: hex bytes not protected from subsequent hex-like chars' \
 				"(expected $expect; got $actual)"
 
+# ======
+# https://github.com/ksh93/ksh/issues/290
+var=dummy
+exp='{}'
+got=$(eval 'echo ${var:+'\''{}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (1)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+unset var
+exp='}'
+got=$(eval 'echo ${var:-'\''}'\''}' 2>&1)
+[[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (2)" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))