caldav: don't force message unlink

This is configurable for the admin already via the delayed_expunge
option - and keeping the files on disk allows for much better
undo/recovery options.

What might be worth considering is a mailbox option flag to turn on
immediate expunge (exposed via metadata like shared-seen) for
calendars with a high churn-rate of edited items.
caldav: MIME encode generated headers

When using user-supplied data to generate message headers, it is
necessary to encode it!
caldav: correct length of mboxname append

sizeof a "char *" variable bugs have happened before, and
been a security bug.

strncat(var, foo, sizeof(var)) is an antipattern.