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_bash3.right b/shell/hush_test/hush-vars/var_bash3.right
new file mode 100644
index 0000000..a97c850
--- /dev/null
+++ b/shell/hush_test/hush-vars/var_bash3.right
@@ -0,0 +1,20 @@
+1 a041#c
+2 a041#c
+3 a\041#c
+4 a\041#c
+5 a\041#c
+6 a\041#c
+7 a\041#c
+8 a\041#c
+9 a\041#c
+10 a\c
+11 a\c
+12 a\c
+13 a\\c
+14 a\\c
+15 a\\c
+16 a\tc
+17 a\tc
+18 a\tc
+19 atc
+20 a\tc