busname: add parser for bus name policies

There are three directives to specify bus name polices in .busname
files:

 * AllowUser [username] [access]
 * AllowGroup [groupname] [access]
 * AllowWorld [access]

Where [access] is one of

 * 'see': The user/group/world is allowed to see a name on the bus
 * 'talk': The user/group/world is allowed to talk to a name
 * 'own': The user/group/world is allowed to own a name

There is no user added yet in this commit.