runtime(sieve): preserve existing line endings in ftplugin

runtime(sieve): preserve existing line endings in ftplugin

Only set fileformat=dos for new files; preserve existing line endings
when editing. This satisfies RFC 5228 for new files while avoiding
issues with version control and existing workflows.

The previous change (3cb4148) unconditionally set fileformat=dos, which
converts existing files with LF line endings to CRLF on save. This
causes issues with version control (entire file appears changed) and
breaks workflows where sieve files are stored with unix line endings.

Dovecot Pigeonhole (the main sieve implementation) has explicitly
accepted LF line endings since 2008 (commit 97b967b5):
  /* Loose LF is allowed (non-standard) and converted to CRLF */
This behavior has remained unchanged for almost 18 years.

closes: #19144

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: André-Patrick Bubel <code@apb.name>
Signed-off-by: Christian Brabandt <cb@256bit.org>