Do not write unprintable characters in XML reports

Convert some unprintable characters to a *plain-text* representation in
XML reports.  This is a little hack but does the trick for now.

The NetBSD test suite has started spitting out a character with value 27,
and that confuses xsltproc when processing an XML-based report created by
atf-report.  Even converting the character to  doesn't seem to work.

NEWS

@@ -30,6 +30,11 @@ STILL UNDER DEVELOPMENT; NOT RELEASED YET.
   executing a test program.  This should help crossbuild systems by
   providing a mechanism to pre-specify what the results should be.
 
+* PR bin/45690: Make atf-report convert any non-printable characters to
+  a plain-text representation (matching their corresponding hexadecimal
+  entities) in XML output files.  This is to prevent the output of test
+  cases from breaking xsltproc later.
+
 
 Changes in version 0.16
 ***********************

atf-report/atf-report.cpp

@@ -31,6 +31,7 @@ extern "C" {
 #include <sys/time.h>
 }
 
+#include <cctype>
 #include <cstdlib>
 #include <fstream>
 #include <iomanip>
@@ -394,17 +395,24 @@ class xml_writer : public writer {
     std::string
     elemval(const std::string& str)
     {
-        std::string ostr;
+        std::ostringstream buf;
         for (std::string::const_iterator iter = str.begin();
              iter != str.end(); iter++) {
-            switch (*iter) {
-            case '&': ostr += "&amp;"; break;
-            case '<': ostr += "&lt;"; break;
-            case '>': ostr += "&gt;"; break;
-            default:  ostr += *iter;
+            const int character = static_cast< unsigned char >(*iter);
+            if (character == '&') {
+                buf << "&amp;";
+            } else if (character == '<') {
+                buf << "&lt;";
+            } else if (character == '>') {
+                buf << "&gt;";
+            } else if (std::isalnum(character) || std::ispunct(character) ||
+                       std::isspace(character)) {
+                buf << static_cast< char >(character);
+            } else {
+                buf << "&amp;#" << character << ";";
             }
         }
-        return ostr;
+        return buf.str();
     }
 
     void