Add utility function dup_wc_to_mb.

This parallels dup_mb_to_wc, which already existed. I haven't needed
the same thing this way round yet, but I'm about to.

misc.h

@@ -67,6 +67,10 @@ void strbuf_finalise_agent_query(strbuf *buf);
  * work around the rather deficient interface of mb_to_wc. */
 wchar_t *dup_mb_to_wc_c(int codepage, int flags, const char *string, int len);
 wchar_t *dup_mb_to_wc(int codepage, int flags, const char *string);
+char *dup_wc_to_mb_c(int codepage, int flags, const wchar_t *string, int len,
+                     const char *defchr, struct unicode_data *ucsdata);
+char *dup_wc_to_mb(int codepage, int flags, const wchar_t *string,
+                   const char *defchr, struct unicode_data *ucsdata);
 
 static inline int toint(unsigned u)
 {

utils/CMakeLists.txt

@@ -17,6 +17,7 @@ add_sources_from_current_dir(utils
   dupprintf.c
   dupstr.c
   dup_mb_to_wc.c
+  dup_wc_to_mb.c
   encode_utf8.c
   encode_wide_string_as_utf8.c
   fgetline.c

utils/dup_wc_to_mb.c

@@ -0,0 +1,39 @@
+/*
+ * dup_wc_to_mb: memory-allocating wrapper on wc_to_mb.
+ *
+ * Also dup_wc_to_mb_c: same but you already know the length of the
+ * string.
+ */
+
+#include <wchar.h>
+
+#include "putty.h"
+#include "misc.h"
+
+char *dup_wc_to_mb_c(int codepage, int flags, const wchar_t *string, int len,
+                     const char *defchr, struct unicode_data *ucsdata)
+{
+    size_t outsize = len+1;
+    char *out = snewn(outsize, char);
+
+    while (true) {
+        size_t outlen = wc_to_mb(codepage, flags, string, len, out, outsize,
+                                 defchr, ucsdata);
+        /* We can only be sure we've consumed the whole input if the
+         * output is not within a multibyte-character-length of the
+         * end of the buffer! */
+        if (outlen < outsize && outsize - outlen > MB_LEN_MAX) {
+            out[outlen] = '\0';
+            return out;
+        }
+
+        sgrowarray(out, outsize, outsize);
+    }
+}
+
+char *dup_wc_to_mb(int codepage, int flags, const wchar_t *string,
+                   const char *defchr, struct unicode_data *ucsdata)
+{
+    return dup_wc_to_mb_c(codepage, flags, string, wcslen(string),
+                          defchr, ucsdata);
+}