libbuxton: Rename buxton_client_get_value function

Because Buxton's get value will not normally use a layer name rename
this function to identify it as using a layer name. We will use the
buxton_client_get_value function name to identify the function that
gets its value from the correct layer based on priority.
libbuxton: Only use direct when BUXTON_SOCKET doesn't exist

In order to prevent database files from being attempted to be used by
the buxton daemon and the client directly, make direct check if the
BUXTON_SOCKET file exists first. If it does exist just use the non
direct open as this works with our existing use cases of testing and
package installation.