bgpd: improve "monotonic" uptime correction

bgpd: improve "monotonic" uptime correction

Older versions of Quagga/Zebra would output a value in MRT table
dump files for "uptime" aka "ORIGINATED" that was a WALL clock
value.  Given that uptime is now internally a bgp_clock MONOTONIC
value, the output in the MRT files is showing up as monotonic.

Note: time of MRT dump is still recorded correctly as a
time() based value, so we haven't lost that value.

Proposal is to correct the uptime output on the vty and in the
MRT files to again display something more akin to WALL time.

* bgp_dump.c: (bgp_dump_routes_func) add conditional correction
* bgp_route.c: (route_vty_out_detail) make correction conditional, move
  variable declaration to beginning of the function