[pkg-horde] Bug#545120: ingo1: uses deprecated imapflags option in Sieve

[Andrew Schulman]

Users of the Dovecot LDA (deliver), as of version 1.2, now have to 
separately enable imapflags by adding

plugin {
  sieve_extensions = +imapflags
}

to their dovecot.conf.  Otherwise, they'll get an error message from ingo1 
about imapflags being unsupported, when they try to save their filter 
scripts.