Drop the inconsistent use of #pragma once.

Apparently it's not totally portable, and the #ifdefs work fine (and
equivalently to the pragma, on some compilers).
Update header comments about how the implementation works.
Fix copyrights on the Makefile.ams.

When initially writing the Makefiles, I started from a Makefile.am
that had ajax's copyright instead of an Intel one.  The only line I
can see that's left from the original Makefile.am is "pkgconfigdir" in
the root Makefile.am.
Add support for lower-priority aliases of functions.

For example, on desktop 2.1 GL on Apple, there's no glBindVertexArray,
but there is glBindVertexArrayAPPLE, and as far as a caller is
concerned, the APPLE variant should be able to stand in for the
core/ARB version.  Similarly for trying to do FBOs on an old Mesa
implementation that didn't have ARB_fbo yet, but did have EXT_fbo.
Update python comments about aliases.
Add and clean up some more comments in the python.

Fixes #8