Propflags: get flag values through getter function

No need for the file and dir classes to manually check if the flags have been
initialized. The getter does that for us.
Propflags: decode full set of smbclient flags

Take the list of flag letters from the smbclient code and make sure we decode
all of them. This might be useful for more advanced Windows clients, because
they get a more fully specified Windows property string for files with special
flags set.
README: note that we don't support Samba 4.x yet