Add CJK codecs support as discussed on python-dev. (SF #873597)

Several style fixes are suggested by Martin v. Loewis and
Marc-Andre Lemburg. Thanks!
diff --git a/Modules/cjkcodecs/_gb2312.c b/Modules/cjkcodecs/_gb2312.c
new file mode 100644
index 0000000..388f0b8
--- /dev/null
+++ b/Modules/cjkcodecs/_gb2312.c
@@ -0,0 +1,69 @@
+/*
+ * _gb2312.c: the GB2312 codec
+ *
+ * Written by Hye-Shik Chang <perky@FreeBSD.org>
+ * $CJKCodecs: _gb2312.c,v 1.2 2003/12/31 05:46:55 perky Exp $
+ */
+
+#include "codeccommon.h"
+
+ENCMAP(gbcommon)
+DECMAP(gb2312)
+
+ENCODER(gb2312)
+{
+    while (inleft > 0) {
+        Py_UNICODE  c = IN1;
+        DBCHAR      code;
+
+        if (c < 0x80) {
+            WRITE1(c)
+            NEXT(1, 1)
+            continue;
+        }
+        UCS4INVALID(c)
+
+        RESERVE_OUTBUF(2)
+        TRYMAP_ENC(gbcommon, code, c);
+        else return 1;
+
+        if (code & 0x8000) /* MSB set: GBK */
+            return 1;
+
+        OUT1((code >> 8) | 0x80)
+        OUT2((code & 0xFF) | 0x80)
+        NEXT(1, 2)
+    }
+
+    return 0;
+}
+
+DECODER(gb2312)
+{
+    while (inleft > 0) {
+        unsigned char    c = **inbuf;
+
+        RESERVE_OUTBUF(1)
+
+        if (c < 0x80) {
+            OUT1(c)
+            NEXT(1, 1)
+            continue;
+        }
+
+        RESERVE_INBUF(2)
+        TRYMAP_DEC(gb2312, **outbuf, c ^ 0x80, IN2 ^ 0x80) {
+            NEXT(2, 1)
+        } else return 2;
+    }
+
+    return 0;
+}
+
+#include "codecentry.h"
+BEGIN_CODEC_REGISTRY(gb2312)
+    MAPOPEN(zh_CN)
+        IMPORTMAP_DEC(gb2312)
+        IMPORTMAP_ENC(gbcommon)
+    MAPCLOSE()
+END_CODEC_REGISTRY(gb2312)