"Bytes left" should be "bytes read"

Actually close the file properly on EOF, even if not aligned to a
sector boundary.  Sigh.

We would incorrectly not set SI=0 when returning from getfssec upon
end of file.  Fix this.

We have made it this far without it, but it seems like a disaster
waiting to happen.  We can afford the extra four bytes in sector 1.

Mechanically rename the writestr functions:

	writestr   -> writestr_early
	cwritestr  -> writestr

... to reflect which function is more appropriate to call in normal
circumstances.

Print the interrupt number, AX, and CS:IP of the offending system
call.

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.