bgpd: work around GCC/plugin shortcomings

bgpd: work around GCC/plugin shortcomings

The GCC extended printf format checking plugin runs into some GCC
shortcomings regarding casts on printf function parameters.  While this
can be fixed with a small GCC patch, patching GCC is "nontrivial" to say
the least.  Luckily, it happens that this is /almost/ not an issue for
the FRR source base.

Since we fix SA "misunderstandings" too, let's just fix places where the
format checking plugin runs into this limitation to keep things working
extra smoothly.

(It's not a huge effort either, these two spots in bgpd are the only
places that trigger the plugin limitation, and it's been "clean" before
that for more than a year if my memory is right.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>