handle families of functions with different signatures

handle families of functions with different signatures

* figl/parse.scm (gl-definition): Change prototype field to prototypes.
  (xml-funcprototypes): New func, replaces xml-prototype.
  (string->gl-type): Trim spaces.  Always make a symbol (what was going
  on here before with the double quote?)
  (parse-prototypes): Extract all prototypes.  Attempt to elide
  redundant procedures.
  (parse-gl-definitions, xml->definition): Don't generate definitions
  for man pages without prototypes.

* maint/update-low-level-bindings (write-scm): Adapt format of
  define-gl-procedure, and generate a correct export list.

* doc/low-level-gl.texi:
* doc/low-level-glu.texi:
* doc/low-level-glx.texi:
* figl/low-level/gl.scm:
* figl/low-level/glu.scm:
* figl/low-level/glx.scm: Update generated files.

* figl/low-level/support.scm (define-gl-procedure): Adapt to make
  multiple definitions.