dos: obtain the executable pathname, cleanups

dos: obtain the executable pathname, cleanups

DOS actually does provide the fully qualified pathname to the
executable, which would be useful to make ldlinux.c32 data rather than
live inside the executable itself -- it has gotten too large.

Also, move some DOS internals -- inline functions only used inside the
dos directory -- out of libinstaller.

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