dovecot: prepare for runtime dependency by plugins

dovecot: prepare for runtime dependency by plugins

Add ABI_VERSION declaration in order to tell dovecot plugins that are
runtime-dependent on dovecot's Application Binary Interface (ABI)
version, such as the pigeonhole mail filter, that they need to be
rebuilt each time the dovecot's ABI version changes.

This PR is not yet closing issue #5062. A forthcoming PR for pigeonhole
package will close the issue.

Introduce a new configuration parameter to enable ICU (i18n) support.
Make the ICU disabled by default.

Introduce PKG_CONFIG_DEPENDS with the list of configuration parameters
triggering rebuild of dovecot.

Declare the conffiles section to save dovecot's configuration files and
its init.d script on sysupgrade.

Introduce a new package, dovecot-utils, to contain doveadm and dsync
utilities: closes #4912.

Correct the destination directory for the installation of
example-config files. At present, the example files are installed into
/usr/share/doc/dovecot/example-config/example-config.

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