estack.eclass: Use 'shopt -p -o' to restore 'set' flags

Use 'shopt -p -o' to obtain the current set of 'set' flags, and to
restore them on popping. Unlike $-, it is output in command form (alike
'shopt -p'), removing the need for separate logic in eshopts_pop.

eclass/estack.eclass

@@ -158,7 +158,7 @@ eshopts_push() {
 		[[ $# -eq 0 ]] && return 0
 		shopt "$@" || die "${FUNCNAME}: bad options to shopt: $*"
 	else
-		estack_push eshopts $-
+		estack_push eshopts "$(shopt -p -o)"
 		[[ $# -eq 0 ]] && return 0
 		set "$@" || die "${FUNCNAME}: bad options to set: $*"
 	fi
@@ -172,12 +172,7 @@ eshopts_push() {
 eshopts_pop() {
 	local s
 	estack_pop eshopts s || die "${FUNCNAME}: unbalanced push"
-	if [[ ${s} == "shopt -"* ]] ; then
-		eval "${s}" || die "${FUNCNAME}: sanity: invalid shopt options: ${s}"
-	else
-		set +$-     || die "${FUNCNAME}: sanity: invalid shell settings: $-"
-		set -${s}   || die "${FUNCNAME}: sanity: unable to restore saved shell settings: ${s}"
-	fi
+	eval "${s}" || die "${FUNCNAME}: sanity: invalid shopt options: ${s}"
 }
 
 # @FUNCTION: eumask_push