Add GlxProto::createEnumFunction and add a 'context' parameter to the

Add GlxProto::createEnumFunction and add a 'context' parameter to the
glXEnumFunction constructor.  The allows sub-classes of GlxProto to
over-ride the concrete class used for glXEnumFunction.

In addition to tracking p_count_parameters in glParameter, break the comma
separated list of parameter names into a Python list called
count_parameter_list.  It is now possible to query if a name is the name of
one of the count parameters just by comparing
param.count_parameter_list.count(n) to zero.  Eventually the remaining uses
of p_count_parameters will be replaced with uses of count_parameter_list.

Make sure that 'void *' parameters are handled correctly in
glParameter::size_string.

Add PrintGlxReqSize_h and PrintGlxReqSize_c.  These classes emit prototypes
and functions used on the server-side to determine the expected size of an
incoming GL command.