because that is what most of the code expects. we could work around having bare
lf, but it would complicate too much code.
currently, a message with bare lf is accepted (in smtpserver delivery,
imapserver append, etc), but when an imap session would try to fetch parsed
parts, that would fail because and even cause a imapserver panic (closing the
connection).
in message imports we would already convert bare lf to crlf (because it is
expected those messages are all lf-only-ending).
we store messages with crlf-ending instead of lf-ending so the imapserver has
all correct information at hand (line counts, byte counts).
found by using emclient with mox. it adds a message to the inbox that can have
mixed crlf and bare lf line endings in a few header fields (in some
localization, emclient authors explained how that happened, thanks!). we can
now convert those lines and read those messages over imap. emclient already
switched to all-crlf line endings in newer (development) versions.
