mdssvc: new RPC service

'mdssvc' aka 'Metadata Search Service' is an RPC service used by Apple
for passing marshalled Spotlight search queries and results between
client to server.

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: add new subsystem for Spotlight

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: add new option 'spotlight'

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: implement async Spotlight RPC

* dalloc: typesafe object container
* Spotlight blob marshalling and unmarshalling
* metadata attribute mapping
* lexer and parser for Spotlight queries
* Spotlight-RPC commands

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: build utility spotlight2sparql

Signed-off-by: Ralph Boehme <rb@sernet.de>