indent: clean up the code, add a few tests

indent: fix spacing between block braces

tests/indent: explain, clean up and add some tests

indent: remove buggy code for swapping tokens

It is not the job of an indenter to swap tokens, even if it's only about
placing comments elsewhere. The code that swapped the tokens was
complicated, buggy and impossible to understand.

In -br (brace right) mode, indent no longer moves a '{' from the
beginning of a line to the end of the previous line, as that was handled
by the token swapping code as well. This change is unintended, but it
will be easier to re-add that now that the code is simpler.

tests/indent: change directive from '#' to '//'

Using a '//' instead of '#' turns the directives into well-formed C
code, resulting in fewer error markers in the editor.

indent: remove FreeBSD IDs

Most of the IDs were empty anyway.

tests/indent: refine and extend tests

tests/indent: migrate a few more tests to t_options