[RFC] Clean up and split 2-step antipattern in set_char_option()

A lot of small changes in the set_char_option() function in option.c (for compliance with the neovim style guide), and the split of the 2-step loop as discussed in issue #572.

There is a lot of commits that might be needless in history. I would like an external opinion on how to squash them. Further refactoring suggestions appreciated as well.
People seem to be in favor of it, so we can incrementally move in that direction.