torcontrol: Handle escapes in Tor QuotedStrings

https://trac.torproject.org/projects/tor/ticket/14999 is tracking an encoding
bug with the Tor control protocol, where many of the QuotedString instances that
Tor outputs are in fact CStrings, but it is not documented which ones are which.

https://spec.torproject.org/control-spec section 2.1.1 provides a future-proofed
rule for handing QuotedStrings, which this commit implements.

src/test/torcontrol_tests.cpp

@@ -119,27 +119,43 @@ BOOST_AUTO_TEST_CASE(util_ParseTorReplyMapping)
             {"Foo", "Bar Baz"},
         });
 
-    // Escapes (which are left escaped by the parser)
+    // Escapes
     CheckParseTorReplyMapping(
         "Foo=\"Bar\\ Baz\"", {
-            {"Foo", "Bar\\ Baz"},
+            {"Foo", "Bar Baz"},
         });
     CheckParseTorReplyMapping(
         "Foo=\"Bar\\Baz\"", {
-            {"Foo", "Bar\\Baz"},
+            {"Foo", "BarBaz"},
         });
     CheckParseTorReplyMapping(
         "Foo=\"Bar\\@Baz\"", {
-            {"Foo", "Bar\\@Baz"},
+            {"Foo", "Bar@Baz"},
         });
     CheckParseTorReplyMapping(
         "Foo=\"Bar\\\"Baz\" Spam=\"\\\"Eggs\\\"\"", {
-            {"Foo", "Bar\\\"Baz"},
-            {"Spam", "\\\"Eggs\\\""},
+            {"Foo", "Bar\"Baz"},
+            {"Spam", "\"Eggs\""},
         });
     CheckParseTorReplyMapping(
         "Foo=\"Bar\\\\Baz\"", {
-            {"Foo", "Bar\\\\Baz"},
+            {"Foo", "Bar\\Baz"},
+        });
+
+    // C escapes
+    CheckParseTorReplyMapping(
+        "Foo=\"Bar\\nBaz\\t\" Spam=\"\\rEggs\" Octals=\"\\1a\\11\\17\\18\\81\\377\\378\\400\" Final=Check", {
+            {"Foo", "Bar\nBaz\t"},
+            {"Spam", "\rEggs"},
+            {"Octals", "\1a\11\17\1" "881\377\37" "8400"},
+            {"Final", "Check"},
+        });
+    CheckParseTorReplyMapping(
+        "Valid=Mapping Bare=\"Escape\\\"", {});
+    CheckParseTorReplyMapping(
+        "OneOctal=\"OneEnd\\1\" TwoOctal=\"TwoEnd\\11\"", {
+            {"OneOctal", "OneEnd\1"},
+            {"TwoOctal", "TwoEnd\11"},
         });
 
     // A more complex valid grammar. PROTOCOLINFO accepts a VersionLine that