This pull request fixes several bugs in `csplit` by rewriting the code
to have one function per pattern type. The functions are

* `up_to_line_loop` for patterns like `123 {45}` and `123 {*}`,
* `up_to_match_pos_offset_repeat` for `/regex/+123 {45}`,
* `up_to_match_pos_offset_always` for `/regex/+123 {*}`,
* `up_to_match_neg_offset_repeat` for `/regex/-123 {45}`,
* `up_to_match_neg_offset_always` for `/regex/-123 {*}`,
* `skip_to_match_pos_offset_repeat` for `%regex%+123 {45}`,
* `skip_to_match_pos_offset_always` for `%regex%+123 {*}`,
* `skip_to_match_neg_offset_repeat` for `%regex%-123 {45}`,
* `skip_to_match_neg_offset_always` for `%regex%-123 {*}`,

The command-line options, global state, and per-chunk state live in the
`Splitter` struct (which replaces the `SplitWriter` and `InputSplitter`
structs). There are a ton of ugly corner cases where different types of
patterns interact; most of these should be called out with comments.

I added some tests that were already passing, but were missing test
coverage, including

* same regex pattern twice in a row, as in `/foo/ /foo/`
* line number out of range on empty input, as in `: | csplit - 1`,
* skip to match with negative offset repeating indefinitely, as in
  `csplit %foo%-5 {*}`.

Some bugs fixed include:

* missing a regex match on the final line of a line number pattern, as
  in `seq 20 | csplit - 10 /10/`,
* missing output `0\n` when input is a directory,
* uutils#7286, a missing empty file with `--suppress-matched`.

Some bugs remain:

* The GNU test file `tests/csplit/csplit-suppress-matched` remains
  failing. I'm hoping that the change in this pull request will make it
  easier to reason about csplit and fix the remaining failures.
* Probably related to the previous item, there is some weird behavior of
  GNU csplit that I could not understand. This was already partially
  covered by a test named `repeat_everything`. I've extracted just the
  relevant part of that into its own test named
  `test_repeat_line_after_match_suppress_matched` that has TODO comments
  for the behavior that differs from GNU `csplit`.

Fixes uutils#7286