hush: implement negative start in the ${v: -n[:m]} idiom

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

shell/ash_test/ash-vars/var_bash6.right

@@ -0,0 +1,5 @@
+Expected Actual
+a*z    : a*z
+\z     : \z
+a1z a2z: a1z a2z
+z      : z

shell/ash_test/ash-vars/var_bash6.tests

@@ -0,0 +1,9 @@
+# This testcase checks globbing correctness in ${v/a/b}
+
+>a1z; >a2z;
+          echo 'Expected' 'Actual'
+v='a bz'; echo 'a*z    :' "${v/a*z/a*z}"
+v='a bz'; echo '\z     :' "${v/a*z/\z}"
+v='a bz'; echo 'a1z a2z:' ${v/a*z/a*z}
+v='a bz'; echo 'z      :' ${v/a*z/\z}
+rm a1z a2z