keepconf is a boring option and is not needed in most situations. Make
this more clear in the documentation, e.g. that it only affects
*unchanged* configuration files, and that changed configuration files
are unaffected by this option.
Add configuration option keepconf that stops xbps from overwriting
unchanged configuration files. If keepconf=true, xbps will store the new
configuration as <name>.new-<version> instead of overwriting unchanged
configuration files.
