FAT: change DOS installer to EXE; additional 32K limit fixes

FAT: change DOS installer to EXE; additional 32K limit fixes

Additional fixes for the 32K limits in the installers.  In the case
of the DOS installer, that means changing it from COM format to EXE
format (since COM format has a 63K hard limit); retain the name
syslinux.com for user compatibility, though (DOS doesn't care what the
extension except for pathname search; if it finds an MZ EXE header it
will use it.)

With the change to EXE means having to handle more than one segment.
Since we don't have a real DOS compiler we have to wing it a bit.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>