Migrate all Python scripts to Python 3.

Most of them are now _mandatory_ P3 scripts, because I'm tired of
maintaining everything to be compatible with both versions.

The current exceptions are gdb.py (which has to live with whatever gdb
gives it), and kh2reg.py (which is actually designed for other people
to use, and some of them might still be stuck on P2 for the moment).

contrib/encodelib.py

@@ -4,14 +4,21 @@
 # The idea of this is that you can use it to manually construct key
 # exchange sequences of interesting kinds, for testing purposes.
 
-import struct, random
+import sys
+import struct
+import random
+
+assert sys.version_info[:2] >= (3,0), "This is Python 3 code"
+
+def tobytes(s):
+    return s if isinstance(s, bytes) else s.encode('ASCII')
 
 def boolean(b):
-    return "\1" if b else "\0"
+    return b"\1" if b else b"\0"
 
 def byte(b):
     assert 0 <= b < 0x100
-    return chr(b)
+    return bytes([b])
 
 def uint32(u):
     assert 0 <= u < 0x100000000
@@ -22,25 +29,24 @@ def uint64(u):
     return struct.pack(">L", u)
 
 def string(s):
-    return uint32(len(s)) + s
+    return uint32(len(s)) + tobytes(s)
 
 def mpint(m):
-    s = ""
-    lastbyte = 0
+    s = []
     while m > 0:
-        lastbyte = m & 0xFF
-        s = chr(lastbyte) + s
+        s.append(m & 0xFF)
         m >>= 8
-    if lastbyte & 0x80:
-        s = "\0" + s
-    return string(s)
+    if len(s) > 0 and (s[-1] & 0x80):
+        s.append(0)
+    s.reverse()
+    return string(bytes(s))
 
 def name_list(ns):
-    s = ""
-    for n in ns:
-        assert "," not in n
-        if s != "":
-            s += ","
+    s = b""
+    for n in map(tobytes, ns):
+        assert b"," not in n
+        if s != b"":
+            s += b","
         s += n
     return string(s)
 
@@ -52,7 +58,7 @@ def ssh_rsa_signature_blob(signature):
 
 def greeting(string):
     # Greeting at the start of an SSH connection.
-    return string + "\r\n"
+    return tobytes(string) + b"\r\n"
 
 # Packet types.
 SSH2_MSG_DISCONNECT = 1
@@ -122,8 +128,5 @@ def decode_uint32(s):
 def read_clearpkt(fh):
     length_field = fh.read(4)
     s = fh.read(decode_uint32(length_field))
-    import sys
-    padlen = ord(s[0])
-    s = s[1:-padlen]
-    msgtype = ord(s[0])
-    return msgtype, s[1:]
+    padlen, msgtype = s[:2]
+    return msgtype, s[2:-padlen]