Add bin object format.

Add bin object format.
Changes to:
 objfmt interface: added parameter to directive() for sections access.
 section interface: moved start to be present for all section types.
 basic optimizer: add section start to offset determination.
(much of the section start changes are due to the need to make sure offsets
don't end up being optimized into byte-sized offsets that can't be relocated).

svn path=/trunk/yasm/; revision=490