Bump mailbox size limit to 512MB

This should hopefully eliminated errors like:
root@olinda:/etc/postfix# head /var/log/mail/dovecot-deliver.log
2017-11-01 06:43:39 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 06:58:13 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 06:58:13 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:19:39 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:19:39 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:20:45 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:20:45 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:31:56 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:31:56 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)
2017-11-01 07:38:38 lda(alex): Error: pwrite_full() failed with file /home/alex/.maildir/.Junk.server/dovecot.index.cache: File too large (process was started with ulimit -f limit)

See also https://wiki.dovecot.org/LDA/Postfix