lib-imap: Add IMAP_ARG_EOL parameter also for unfinished lists

lib-imap: Add IMAP_ARG_EOL parameter also for unfinished lists

If list parsing was stopped due to IMAP_ARG_LITERAL_SIZE* the list didn't
have IMAP_ARG_EOL added. However, imap_arg_get_list() could still have
been used for it, which assumes that the IMAP_ARG_EOL exists.

This didn't seem to cause any actual bugs so far, but the following commits
require this.

The IMAP_ARG_EOL is appended and deleted immediately. This way it's seen by
imap_arg_get_list(), but it won't change any other behavior unexpectedly.