SICS-764: Provide access to the asynnet socket descriptor's write buffer.

We need this to implement a "capture" command that doesn't cause SICS to
commit suicide if you capture a "run" command.
NOTE: This is a bit dodgy because we're trusting callers not to modify the write buffer.

asynnet.c

@@ -632,3 +632,14 @@ int ANETreadTillTerm(int handle,
   }
   return ANETTIMEOUT;
 }
+
+char *ANETgetWBuffer(int handle) {
+  pSocketDescriptor sock;
+  int len;
+  char *wdata;
+
+  sock = findSocketDescriptor(handle);
+  wdata = (char *) GetRWBufferData(sock->writeBuffer, &len);
+  wdata[len] = '\0';
+  return wdata;
+}