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

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

shell/hush_test/hush-vars/var_bash1a.right

@@ -0,0 +1,6 @@
+parameter     'abcdef'
+varoffset2    'cdef'
+varoffset-2   'ef'
+literal '2'   'cdef'
+literal '-2'  'abcdef'
+literal ' -2' 'ef'

shell/hush_test/hush-vars/var_bash1a.tests

@@ -0,0 +1,11 @@
+parameter=abcdef
+offset=2
+noffset=-2
+echo "parameter     '${parameter}'"
+echo "varoffset2    '${parameter:${offset}}'"
+echo "varoffset-2   '${parameter:${noffset}}'"
+echo "literal '2'   '${parameter:2}'"
+# This is not inrpreted as ${VAR:POS{:LEN}},
+# but as ${VAR:=WORD} - if VAR is unset or null, substitute WORD
+echo "literal '-2'  '${parameter:-2}'"
+echo "literal ' -2' '${parameter: -2}'"