Fix ERE repetition expressions in [[ ... =~ ERE{x,y} ]] (#54)

Regular expressions that combine a repetition expression with
a parenthesized sub-expression throw a garbled syntax error:

$ [[ AATAAT =~ (AAT){2} ]]
ksh: syntax error: `~(E)(AAT){2} ]]
:'%Cred%h%Creseksh: syntax error: `~(E)(AAT){2} ]]
:'%Cred%h%Creseksh: syntax' unexpected

The syntax error occurs because ksh is not fully
accounting for '=~' when it runs into a curly bracket.
This fix disables the syntax error when the operator
is '=~' and adds handling for '(str){x}' (to allow for
more than one sub-expression). This bugfix and the
regression tests for it were backported from ksh93v-
2014-12-24-beta.

src/cmd/ksh93/sh/lex.c:
- Do not trigger a syntax error for '{x}' when the operator
  is '=~' and add handling for multiple parentheses when
  combined with '{x}'.

src/cmd/ksh93/tests/bracket.sh:
- Add two tests from ksh93v- to test sub-expressions
  combined with the '{x}' quantifier.

src/cmd/ksh93/tests/bracket.sh

@@ -388,5 +388,10 @@ var=$(echo begin; exec >/dev/tty; [ -t 1 ] && test -t 1 && [[ -t 1 ]]) \
 test 123 -eq 123x 2>/dev/null
 [[ $? -ge 2 ]] || err_exit 'test builtin should return value greater than 1 on error'
 
+# ======
+# The '=~' operator should work with curly brackets
+$SHELL -c '[[ AATAAT =~ (AAT){2} ]]' || err_exit '[[ AATAAT =~ (AAT){2} ]] does not match'
+$SHELL -c '[[ AATAATCCCAATAAT =~ (AAT){2}CCC(AAT){2} ]]' || err_exit '[[ AATAATCCCAATAAT =~ (AAT){2}CCC(AAT){2} ]] does not match'
+
 # ======
 exit $((Errors<125?Errors:125))