smack: check for smackfs first

When Smack support is not enabled, a valid SMACK_LOAD_FILE is not
sufficient to track Smack support in bt-daemon, since SMACK_LOAD_FILE is
redefined to test/test.load2 for tests.

Instead, check the absolute directory of the smackfs first, and set
'have_smack' appropriately.
smack: add accessor for Smack state

Use the accessor for one of the test cases that fails when Smack is not
enabled on the system.