Config: update config system, config files now supply arrays

Previously, SambaDAV was configured by supplying global variables and defining
constants. Lose the dependence on global state and change to a more flexible
system where each file in /config supplies an array with config variables that
is merged in.
README.md: update the section about config to reflect changes
README.md: SambaDAV is now also known to work with smbclient-v4