rename variables for better readability

The naming of variables in this function is confusing, and really threw
me for a loop: sec is first used as an iterator, then sec is reused to
point to the dynrela section, then sec2 is used as another iterator.

Instead make sec the iterator for both loops and dynsec the dynrela
section pointer.
Merge pull request #383 from jpoimboe/sec2-var-rename

rename variables for better readability