*: replace all random() calls

*: replace all random() calls

Replace all `random()` calls with a function called `frr_weak_random()`
and make it clear that it is only supposed to be used for weak random
applications.

Use the annotation described by the Coverity Scan documentation to
ignore `random()` call warnings.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>