Renamed unprefixed LOADER typedef.

Types declared in public headers should have a prefix.  The typedef has
been renamed to the more OpenGL-like GLADloadproc.
Renamed function pointer prefixes.

The client API function pointer typedefs have been renamed to the
standard PFN<function>PROC pattern.  An underscore has been added
between the glad prefix and the client API function name to make it
easier and quicker to tell where one ends and the other begins.
Enabled loading of no extensions.
Removed unused GLAPI macro.

This macro is commonly used to introduce directives like
__declspec(dllimport), which are not relevant for function pointers
exported from an always static library like glad.  It's better to avoid
using this potentially overridden macro and just use extern.