Begin the process of centralizing configurables.

Improve "make install" and "make netinstall"; add "make
extbootinstall".

Move source files out of the root directory; the root is a mess and
has become virtually unmaintainable.  The Syslinux core now lives in
core/; the Linux and generic utilities has moved into utils/, and
copybs.com has moved into dos/; it had to go somewhere, and it seemed
as good a place as any.

Remove reference to readpit.c32, an experimental module never intended
for release.

Use $(CC) in gcc_ok macro, not plain gcc.  This seems to work, iff the
gcc_ok macro is declared with =, not :=

Use "-nostdinc -iwithprefix include" to prevent the inclusion of
system header files, but still permitting *compiler* header files.

Make "Hello, World!" a little bit more plain, but do print out its
arguments, if any.  Useful for testing.

Conflicts:

	extlinux/extlinux.c