lib-sieve: include extension: didn't get semantics of mixing optional and non-optional includes for the same script quite right.

Change backported from Pigeonhole v0.4 tree, which means that this change will duplicate there upon merge.

Added support for the :optional tag of the include command.
Changed Sieve script API in the process; opening the script is now available as a separate step.

This was broken by the ld.gold fix.

Those version numbers are now also stored in the binary, making sure it is recompiled
when a new version of an extension is used. This avoids updating the binary version
for minor changes to extensions and also makes this work for plugins.

sieve-refilter: improved man page documentation by explicitly specifying the syntax used for mailbox arguments.

Testsuite failed with an assertion, because the stream returned -2 at an unexpected time.
I've restructured the buffering implementation to prevent this.
The testsuite succeeds now, but this needs to be tested more thoroughly.

Also contains a few other small fixes by Timo.

Before, sievec and sieve-dump would switch to mail_uid:mail_gid and then fail to compile/dump a root script.

Also fixed a small mistake in the spefication for the vnd.dovecot.duplicate extension.

lib-sieve: increase action instance count (for limit checking) only when an action is actually created.