hush: add support for ${var/pattern/repl}, conditional on bash compat

function                                             old     new   delta
expand_vars_to_list                                 2386    2833    +447
expand_string_to_string                               69     110     +41
parse_dollar                                         681     721     +40
hush_main                                            963     945     -18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 528/-18)           Total: 510 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/shell/hush_test/hush-vars/var_bash1.tests b/shell/hush_test/hush-vars/var_bash1.tests
new file mode 100755
index 0000000..24d3c9a
--- /dev/null
+++ b/shell/hush_test/hush-vars/var_bash1.tests
@@ -0,0 +1,18 @@
+var=abcdef
+
+echo ${var:7}
+echo ${var:6}
+echo ${var:5}
+echo ${var:1}
+echo ${var:0}
+echo ${var:-1}
+
+echo ${var:1:4}
+echo ${var:0:4}
+echo ${var::4}
+echo ${var:-1:4}
+
+echo ${var:1:7}
+echo ${var:0:7}
+echo ${var::7}
+echo ${var:-1:7}