Get rid of the shipped binary APKs about mailu

[kaiyou]

Discussion with ncopa from Alpine on IRC, that will help getting rid of the shipped antispam plugin:

<ncopa> re dovecot
<ncopa> the plugins needs to be rebuilt for every dovecot update
<kaiyou_> not every update, but mostly yes, everytime the ABI number is bumped
<ncopa> which means that we could in theory build the plugins with the dovecot package
<ncopa> i think the module checks the minor version number

[kaiyou]

One last binary to go, we will be ok as soon as the extdata plugin is accepted into Alpine.

[kaiyou]

Setting this as blocking for release 1.4, I will push my patches to Alpine during the week.

[kaiyou]

The patch was pushed for extdata : alpinelinux/aports#571

Regarding antispam, there is an ongoing discussion on the Dovecot mailing list stating the Antispam plugin does not seem to be maintained anymore. A proposed solution is to use the latest IMAPSieve features with Extprograms to run the training binary from Sieve : http://www.dovecot.org/list/dovecot/2016-November/106285.html

It sounds reasonable given the nightmare that is building the antispam plugin properly (custom build system, no libtool, etc.)