Closes #5062. With ABI_VERSION declaration in dovecot's Makefile
introduced in PR #5182, pigeonhole plugin will now be rebuilt on every
dovecot upgrade. To announce the rebuilt pigeonhole plugin to opkg,
introduce a different model of version numbering. The new version
numbering now includes both dovecot and pigeonhole plugin version
numbers. Therefore, the rebuilt pigeonhole plugin will be visible to
opkg whenever dovecot or pigeonhole plugin version changes.

Rename "pigeonhole" to "dovecot-pigeonhole". There are several reasons
for renaming the package:

1. pigeonhole is a plugin. There seems to be a convention to name
plugins by adding corresponding suffixes to the parent package name,
such as lua-*, luci-*, php7-mod-*, perlbase-*, transmission-* etc.

2. When named as "dovecot-pigeonhole", the pigeonhole plugin is
displayed right next to dovecot in config manager ("make menuconfig")
and is therefore highly visible to anyone who explores the available
modules for dovecot.

3. When named as "dovecot-pigeonhole", the package version number
consisting of both dovecot and pigeonhole version numbers makes more
sense: "dovecot-pigeonhole_2.2.33.2-0.4.20-2" looks more comprehensible
than "pigeonhole_2.2.33.2-0.4.20-2", however admittedly both variants
look rather ugly.

Add ManageSieve to the package. Adding managesieve-login and its
libraries increases the size of the package by about 11% from 702061
bytes to 781294 bytes on the bcm53xx platform. As dovecot is unlikely
to be used on a low-end LEDE/OpenWrt box, the addition of ~80KB to
the package size is not likely to make a noticeable difference but it
will add the capability to use pigeonhole plugin for dovecot with
systems that talk ManageSieve, such as the RoundCube.

Signed-off-by: Val Kulkov <val.kulkov@gmail.com>