cleanup: make --force less aggressive

Change behavior for `brew cleanup` as follows:

- If `--force` is supplied, remove only outdated keg-only packages.
- If `--prune=<days>` is supplied, remove both logs and cached downloads
  older than the specified number of days. Use `--prune=all` to remove
  all logs and cached downloads irrespective of age.
- By default, remove logs after 14 days and cached downloads never.

Also centralizes handling of `--prune`, thus removing duplicate logic.

This is motivated by commit 17eee232838d4639b25f863aa342b1dda61b81bc
that made `--force` much more aggressive and made it override whatever
was specified via `--prune`, completely removing all:

- outdated keg-only packages
- cached downloads irrespective of age
- logs irrespective of age

This made it impossible to remove outdated keg-only packages without
also deleting all cached downloads, which is at least inconvenient for
people with limited bandwidth wanting to rebuild packages later.

Closes Homebrew/homebrew#42970.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>