add 4 byte alignment for the section table

this is probably not required but definitely does not hurt
fix alignment of segments during saving -> copied elfs are working

The old implementation aligned the segment start. However, the
intended behaviour is to align the offset and the vaddr
(p_vaddr % p_align == p_offset % p_align). This is required for the
loader which can then operate on memory pages.
initialized a few variables to silence warnings when building with Wall
use the NULL section instead of offset == 0

This change makes it possible to use the regular API to build up
elf files which include the file header.
Merge pull request #7 from niosHD/fix-and-cleanup-saving

Fix and cleanup saving