rc lint: Add a lint-selections command to lint part of a buffer.

Fixes #2302, #3336.

Addresses parts of #3155.

Changes include:

- New `lint-selections` command that only lints the current selections,
  and allows a custom lint command.
- New `lint-buffer` command that always lints the whole buffer with
  the linter specified in the lintcmd option.
- `lint` alias for `lint-buffer`, for backwards compatibility.
- Errors and warnings are now shown in the Error and Information faces,
  not hard-coded red and yellow.
- Error and warning flags now use "!" and "?" symbols respectively,
  instead of a unicode block, so they can still be distinguished
  in a monochrome colour-scheme or by colour-blind users.
- An error flag on a given line always takes precedence over a warning.
- All messages for the same line are collected into a multi-line message.
- We no longer escape tildes in messages, since that change was added
  in commit ae339dc (2016) when we started using `%~~` to quote messages.
  We stopped using `%~~` in commit 1a2eecd (2018).
- Anything the linter writes to stderr is logged to the *debug* buffer,
  not lost.
- If the linter writes to stderr, an error is shown to the user instead
  of the usual error/warning count.
- The `lint_errors` hidden option is replaced by `lint_messages`,
  because it contains warnings as well as errors.
- `lint-next-error` renamed to `lint-next-message`,
  and `lint-previous-error` renamed to `lint-previous-message`
  for the same reason.
    - New `lint-next-error` and `lint-previous-error` aliases,
      for backwards compatibility.
- `lint-next-message` and `lint-previous-message` show the message
  they jump to.
- Where `lint_errors` was a range-specs option, `lint_messages` is a
  line-specs option to keep things simpler. This means lint-next-message
  and lint-previous-message no longer jump to a specific column.

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