Centralize more (most) of the version number machinery to version.pl.

Clean up the BSUBDIR/ISUBDIR separation further.  Build _bin.c files,
which are an intermediate stage toward building the installers, in the
libinstaller directory, since that directory is used by all the
installers anyway.  That also lets us get bin2c.pl out of the root.

- "make clean" should not clean up things that "make installer" cannot
  regenerate.
- "make installer" should not descend into subdirectories which are
  target only, e.g. com32 and sample.

The Linux platform stuff in com32 is old and bitrotted, and was only
for debugging in the first place.  Just ignore it.

Add support for creating codepage/Unicode mapping tables; this is a
step toward VFAT long filename support.

Begin the process of centralizing configurables.

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

The only things we still have in the root is version.h and
version.gen.  As a result, move all the subdirectories later, since
they are common prerequisites.

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.

Assemble all files to ELF instead of using the NASM built-in linker.
This can enable us to do actual linking in the future.