[dovecot/core] affded: doveadm save: Fix using <file without panic

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: affded869cee02bd59561f75cb478134f5ee4adf
      https://github.com/dovecot/core/commit/affded869cee02bd59561f75cb478134f5ee4adf
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2018-04-24 (Tue, 24 Apr 2018)

  Changed paths:
    M src/doveadm/doveadm-mail.c

  Log Message:
  -----------
  doveadm save: Fix using <file without panic

When getting input with <file with Linux/epoll, it crashed with:

Panic: epoll_ctl(add, 0) failed: Operation not permitted (fd doesn't support epoll - instead of '<file', try 'cat file|')