Explain frag alignment hacks

"the weird alignment hackery" comment doesn't help anyone understand
the code.  Explain what is going on.  Replace the zero length
obstack_alloc with obstack_finish, which by inspection of obstack.h is
all the zero length alloc does.

	* frags.c (frag_alloc): Comment.  Replace zero length
	obstack_alloc with obstack_finish.
	(frag_new): Remove unnecessary obstack_finish.
	* write.c (compress_frag, compress_debug): Likewise.