Added __glExtensionBiIsEnabled and __GLXcontext::gl_extension_bits. This

Added __glExtensionBiIsEnabled and __GLXcontext::gl_extension_bits.  This
enables libGL to query which extension are exported to applications.

Refactored array-query functionality (from glGet*v) in src/glx/x11/single2.c.

Massive re-write of indirect vertex array support.  The most noticable
effect is that glDrawElements now generates DrawArrays protocol.  The
side-effects (and the main reasons for the re-work) are that it is much
easier to add support for new arrays (e.g., GL_VERTEX_ATTRIB_ARRAY,
GL_WEIGHT_ARRAY_ARB, etc.) and it is much easier to add support for the new
DrawArrays protocol (required to support ARB_vertex_buffer_object).

These changes were primarilly tested with progs/demos/isosurf.