daemon: store client Smack labels as BuxtonStrings

Since BuxtonString was introduced, it's more appropriate to use
BuxtonStrings to handle the Smack label strings.
Use BuxtonString for smack access checks

Similarly here, the daemon will be calling buxton_check_smack_access()
in a few places, so the relevant parameters should be BuxtonStrings.
Remove BuxtonSmackLabel type
Add initial support for Smack access checking

Buxton currently needs to check access for every non-direct get/set on a
key, implemented by this commit.

In the near future, additional checks will be needed for group
creation/modification once groups are implemented.