dovecot: allow reading /proc/sys/fs/suid_dumpable
This is needed if a dovecot child process segfaults - in this case, dovecot provides a helpful error message like
dovecot: auth-worker: Fatal: master: service(auth-worker): child 8103 killed with signal 11 (core not dumped - https://dovecot.org/bugreport.html#coredumps - set /proc/sys/fs/suid_dumpable to 2)
which involves reading the current value in suid_dumpable.
I propose this fix for 2.10..master.