Cleaned up nomenclature in the parser.

Replace the generic "expression" with "condition".
store, frontend: Renamed sdb_store_lookup() to sdb_store_scan().

It does a full scan of the entire store, so lookup sounds a bit euphemistic.
Merged branch 'master' of git://git.tokkee.org/sysdb.
Query language: Changed 'LOOKUP .. WHERE' to 'LOOKUP .. MATCHING'.

'WHERE' was too SQL-like while it has somewhat different semantics. Using a
different keyword will hopefully avoid any confusion.
Merged branch 'master' of git://git.tokkee.org/sysdb.

Conflicts:
	src/frontend/grammar.y