Add a manual single-char UTF-8 decoder.

This parallels encode_utf8 which we already had.

Decoding is more fraught with perils than encoding, so I've also
included a small test program.

utils/decode_utf8_to_wchar.c

@@ -0,0 +1,20 @@
+/*
+ * Decode a single UTF-8 character to the platform's local wchar_t.
+ */
+
+#include "putty.h"
+#include "misc.h"
+
+size_t decode_utf8_to_wchar(const char **utf8, wchar_t *out)
+{
+    size_t outlen = 0;
+    unsigned wc = decode_utf8(utf8);
+    if (sizeof(wchar_t) > 2 || wc < 0x10000) {
+        out[outlen++] = wc;
+    } else {
+        unsigned wcoff = wc - 0x10000;
+        out[outlen++] = 0xD800 | (0x3FF & (wcoff >> 10));
+        out[outlen++] = 0xDC00 | (0x3FF & wcoff);
+    }
+    return outlen;
+}