Fix a few 'triple letter in place of double' typos.

A user wrote in to point out the one in winhandl.c, and out of sheer
curiosity, I grepped the whole source base for '([a-zA-Z])\1\1' to see
if there were any others. Of course there are a lot of perfectly
sensible ones, like 'www' or 'Grrr', not to mention any amount of
0xFFFF and the iiii/bbbb emphasis system in Halibut code paragraphs,
but I did spot one more in the recently added udp.but section on
traits, and another in a variable name in uxagentsock.c.

windows/winhandl.c

@@ -201,7 +201,7 @@ static DWORD WINAPI handle_input_threadfunc(void *param)
 }
 
 /*
- * This is called after a succcessful read, or from the
+ * This is called after a successful read, or from the
  * `unthrottle' function. It decides whether or not to begin a new
  * read operation.
  */