Add PIE and RELRO to binaries

MODULE_LIBS is now part of LIBDOVECOT. No need to explicitly link it everywhere.

--without-shared-libs: Link Dovecot libraries with --whole-archive flag for binaries. This removes the need for the ugly unused_objects lists for binaries, which were needed to avoid plugins from failing because they were missing some functions. Apparently there's no easy way to use --whole-archive properly with libtool, so there's now a rather ugly cc-wrapper.sh that does it. Also this is done only when GNU ld is deted. Most people are going to use --with-shared-libs though, so I don't think any of this is going to be a real problem.

lib-storage: Added mailbox_get_path_to() wrapper for mailbox_list_get_path(). Changed the code to use as much of mailbox_get_path_to() and mailbox_get_path() as possible.

Makefile: Removed mountpoint.o from unused_objects, since it's now included via lib-master.

Makefile: Link with SSL_LIBS whenever linking libssl_iostream.la

indexer: Fixed loading some plugins when using configure --without-shared-libs.

Added "indexer" service, which uses worker processes to perform queued mailbox indexing. Only a single worker process will index the same user at the same time. This avoids lock waits, especially when doing full text search indexing with backends that require locking.