isisd: add the "show isis fast-reroute summary" command

isisd: add the "show isis fast-reroute summary" command

Add new "show" command to make it easy to see the protection coverage
provided by LFA/rLFA/TI-LFA.

Example output:

debian# show isis fast-reroute summary
Area 1:
 IS-IS L1 IPv4 Fast ReRoute summary:

 Protection \ Priority     Critical  High      Medium    Low       Total
 --------------------------------------------------------------------------
 Classic LFA               0         0         1         3         4
 Remote LFA                0         0         0         0         0
 Topology Independent LFA  0         0         0         0         0
 ECMP                      0         0         0         0         0
 Unprotected               0         0         2         1         3
 Protection coverage       0.00%     0.00%     33.33%    75.00%    54.17%

 IS-IS L1 IPv6 Fast ReRoute summary:

 Protection \ Priority     Critical  High      Medium    Low       Total
 --------------------------------------------------------------------------
 Classic LFA               0         0         1         0         1
 Remote LFA                0         0         0         0         0
 Topology Independent LFA  0         0         0         0         0
 ECMP                      0         0         0         0         0
 Unprotected               0         0         2         0         2
 Protection coverage       0.00%     0.00%     33.33%    0.00%     33.33%

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>