Slightly modify the meaning of the 'handcode' attribute in a 'glx' element.

Slightly modify the meaning of the 'handcode' attribute in a 'glx' element.
The attribute can now take one of 4 states.  "false" (the default value)
means that no handcoding is required for the function.  "client" means that
the function must be handcoded on the client-side only.  "server" means that
the function must be handcoded on the server-side only.  "true" menas that
the function must be handcoded on both the client-side and the server-side.

Version 1.14 of glX_proto_send.py accidentally contained a line of this
change.