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>

shell/hush_test/hush-vars/var_bash3.tests

@@ -0,0 +1,41 @@
+a='abc'
+r=${a//b/\041#}
+echo 1 $r
+echo 2 ${a//b/\041#}
+echo 3 "${a//b/\041#}"
+
+a='abc'
+r=${a//b/\\041#}
+echo 4 $r
+echo 5 ${a//b/\\041#}
+echo 6 "${a//b/\\041#}"
+
+a='abc'
+b='\041#'
+r=${a//b/$b}
+echo 7 $r
+echo 8 ${a//b/$b}
+echo 9 "${a//b/$b}"
+
+a='abc'
+b='\'
+r="${a//b/$b}"
+echo 10 $r
+echo 11 ${a//b/$b}
+echo 12 "${a//b/$b}"
+
+a='abc'
+b='\\'
+r="${a//b/$b}"
+echo 13 $r
+echo 14 ${a//b/$b}
+echo 15 "${a//b/$b}"
+
+a='abc'
+b='\t'
+r="${a//b/$b}"
+echo 16 $r
+echo 17 ${a//b/$b}
+echo 18 "${a//b/$b}"
+echo 19 ${a//b/\t}
+echo 20 "${a//b/\t}"