Update candidate RP configuration support and documentation

- Add upper boundary check of cand_rp interval time, using Cisco MAX 16384
- Add support for configuring a cand_rp using the interface name like phyint
- Update man-page about MAX/MIN of priority and interval, and new ifname opt

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Update RP address, remove priority field, should always be 1.

The rp_address priority field was unused since its introduction in
commit 0603f191a.  This commit reverts it since static RP's have
always been announced with priority 1, this is the same value as
Juniper uses and neither Juniper or Cisco have the priority of
static RP's configurable.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Followup to update cand_rp, also check MAX prio boundary.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Update candidate BSR configuration support and documentation

- Add support for configuring a cand_bootstrap_router with interface name
- Update man-page and default pimd.conf

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
config.c: Minor cleanup.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
README: Major update, to be rebased and merged with other README commits...

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