Upgrade mail/dovecot2-pigeonhole to 0.4.18.

Changelog v0.4.18:

+ imapsieve plugin: Implemented the copy_source_after rule action. When

this is enabled for a mailbox rule, the specified Sieve script is
executed for the message in the source mailbox during a "COPY" event.
This happens only after the Sieve script that is executed for the
corresponding message in the destination mailbox finishes running
successfully.

+ imapsieve plugin: Added non-standard Sieve environment items for the

source and destination mailbox.

• multiscript: The execution of the discard script had an implicit "keep", rather than an implicit "discard".

Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D10366