doc/example-config: Added description for a few generic settings that were only mentioned in the INSTALL file.

Forgot to adjust a few things when the location types were introduced and documented.

It is still a plugin, but no longer a separate package.

This currently breaks uninstall when the settings plugins are removed and 20-managesieve.conf
remains in conf.d directory. These sections only need to be active in the config file when settings
for the services themselves need to be disabled.

Additional paths are specified by adding a sequence number (starting at 2) to the setting name.
Added support for specifying home-relative paths for sieve_before and sieve_after.
This enables administrators to specify user-specific global Sieve scripts that cannot be viewed or changed by virtual users.

Old name is now deprecated.
Support for the even older global_script_path name for this setting is dropped.

Setting sieve_max_redirects=0 now means that redirects are prohibited rather than unlimited.
Now there is always a limit, but it can be enormous if the administrator wants to.

Improved ManageSieve config example; 'sieve' is added to protocols setting in 20-managesieve.conf now.