hush: improve ${var#...}, ${var:+...} and ${var/.../...} - handle quoting

dollar_altvalue1 test partially fails: word splitting of unquoted ${var:+...}
is not correct

function                                             old     new   delta
encode_then_expand_vararg                              -     443    +443
expand_one_var                                      1599    1610     +11
parse_stream                                        2756    2753      -3
encode_string                                        250     242      -8
setup_heredoc                                        308     298     -10
expand_and_evaluate_arith                            106      96     -10
encode_then_expand_string                            142     126     -16
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/5 up/down: 454/-47)           Total: 407 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-z_slow/many_ifs.tests

@@ -229,8 +229,8 @@ do
 						'')	split "$d0$f1$d1$f2$d2$f3$d3" "[2]($f1)($f2)" "($f1)($f2)" ;;
 						' ')	;;
 						*)	x=$f2$d2$f3$d3
-							x=${x# } #was x=${x#' '} hush needs fixing for this to work
-							x=${x% } #was x=${x%' '}
+							x=${x#' '}
+							x=${x%' '}
 							split "$d0$f1$d1$f2$d2$f3$d3" "[3]($f1)($f2)($f3)" "($f1)($x)"
 							;;
 						esac