bgpd: random() returns long

bgpd: random() returns long

bgpd was using unsigned to store a probability value to be used with
random().  That, however, returns long, running into some warnings (and
worst case, if RAND_MAX > UINT_MAX, won't work correctly.

Just use long to shuffle the value around.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 8c9cd85631b77fac0bc30ffb9f23b29c466d31c4)