mbox: If write fails with EFBIG, give an error that mentions ulimit -f.
mbox: If write fails with EFBIG, give an error that mentions ulimit -f.

--HG--
branch : HEAD

jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch