Tests and cleanups for existing code, no new functionality.

* lispmud.asd: added "tables.lisp" to system defintion FiveAM depency.

* src/command.lisp: Minor changes

* src/core-command.lisp: Bugfix of word-dispatch, tests with FiveAM added.

* src/input-handlers.lisp: Function renamed.

* src/tables.lisp: No error if dropping nonexistent SQL tables.

* src/userdb.lisp: *online-users* dynamic variable was removed. Now
all checking of online users is done via SQL query.