Pigeonhole Sieve LDAP support

[Stephan Bosch]

Hi,

Yesterday, I committed two important and rather big changes to the 
Pigeonhole repository.

The first creates a generic interface for implementing Sieve script 
storages. It was already possible to some extent to read Sieve scripts 
from other sources than the local filesystem, such as Dovecot dict, but 
that was still pretty limited, as it was not possible to do this with 
sieve_before/sieve_after. This is now supported. But, more importantly, 
ManageSieve did not support storing Sieve scripts on locations other 
than the local filesystem. Now, alternatives can be implemented. Still, 
I haven't actually implemented write access to a storage other than the 
local filesystem yet, but now this would be much easier to achieve.

The second creates an LDAP script storage implementation. Much like the 
dict storage, it is still read-only. It can be compiled directly into 
the Sieve interpreter, and, alternatively, it can also be compiled as a 
plugin called sieve_storage_ldap.

I've updated the documentation in the sources. For all Xi repository 
users: I added compilation of LDAP storage as a plugin. Read the INSTALL 
files and other files referenced therein for more information about how 
to use the plugin. The plugin is installed as part of the dovecot-ldap 
package.

This is a rather big change, but it should all be backwards compatible 
in terms of configuration.

If you have problems (I heard about one so far), please don't hesitate 
to notify me.

Regards,

Stephan.