Add class parameter dns::group_manage which defaults to true, so that
the module keeps managing the system group for BIND ("bind" or "named"),
but allow the user to set it to false. This way the group can be managed
outside of this module's scope.

Debian and RedHat allow setting startup options and other settings via a
sysconfig/default file (/etc/default/bind9 under Debian and
/etc/sysconfig/named under RedHat). FreeBSD/DragonFly and ArchLinux
don't have a direct equivalent.

This change allows managing sysconfig/default settings under Debian and
RedHat. It is especially useful for setting startup options like `-4` to
disable IPv6 support in named, or setting the debug level with `-d`. The
change adds a few specific `$dns::sysconfig_*` parameters to model the
default behaviour under Debian and RedHat, as well as a
`$dns::sysconfig_additional_settings` hash parameter that allows setting
arbitrary key-value pairs in the sysconfig file.

For FreeBSD/DragonFly and ArchLinux these parameters are all set to
undef and not used anywhere since they feature is no directly equivalent
sysconfig setup.

This change also adds basic spec tests for Debian, including tests for
the sysconfig support introduced here.