Fix issue #31: Make IGMP query interval and querier timeout configurable.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Update pimd.conf and pimd(8) with new configuration options

    default_igmp_query_interval <SEC>
        Time between IGMP queries, when elected.
        Default: 125 sec

    default_igmp_querier_timeout <SEC>
        Time before currently elected querier times out.
        Default: 255 sec

    spt_threshold [rate <KBPS> | packets <NUM> | infinity] [interval <SEC>]
        Replaces old configuration options, which are kept for backwards
        compatibility.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>