config: add utility function vdgoption_bool()

Signed-off-by: Ralph Boehme <rb@sernet.de>
Writing metadata xattr on directories with sticky bit set, FR#94

Even though we may have write access to a directory, if the sticky bit
is set only the owner is allowed to write xattrs.

Add an option that enables behaviour to set the xattr as root, using
the fact that we can distinguish between a failing xattr set request
due to insufficient permissions (EACCES) or the sticky bit (EPERM):

  force xattr with sticky bit = yes|no (default: no)

Signed-off-by: Ralph Boehme <rb@sernet.de>