zstring: fix encode rountrip for '\' as printable ASCII (#5120)

This fixes encode roundtripping for all printable ASCII characters.
In particular, this now leaves a plain '\' untouched by the
encoding logic, instead of converting it to escaped hex-digits.
It also adds unit testing covering this specific zstring encoding
property, in order to avoid future regressions.

src/test/zstring.cpp

@@ -0,0 +1,25 @@
+#include "util/debug.h"
+#include "util/trace.h"
+#include "util/zstring.h"
+
+// Encode and check for roundtrip all printable ASCII characters.
+static void tst_ascii_roundtrip() {
+    unsigned ascii_min = 0x20; // ' '
+    unsigned ascii_max = 0x7E; // '~'
+
+    for (unsigned i = ascii_min; i <= ascii_max; i++) {
+        zstring input(i);
+        std::string expected(1, i);
+        bool roundtrip_ok = input.encode() == expected;
+
+        if (!roundtrip_ok) {
+            std::cout << "Failed to roundtrip printable ASCII char: " << expected
+                      << "\n" << std::flush;
+            ENSURE(roundtrip_ok);
+        }
+    }
+}
+
+void tst_zstring() {
+    tst_ascii_roundtrip();
+}