Also now uses CMake's ENABLE_EXPORTS target property for the zeek
executable to ensure symbols are visible to plugins. Prior to CMake
3.4, the policy was to export symbols by default for certain platforms,
but later versions need either the explicit target property or policy.
In deterministic mode, RAND_MAX is not related to the result of
zeek::random_number() (formerly bro_random()), but some logic was
using RAND_MAX as indication of the possible range of values. The
new zeek::max_random() will give the correct upper-bound regardless
of whether deterministic-mode is used.
This unfortunately cuases a ton of flow-down changes because a lot of other
code was depending on that definition existing. This has a fairly large chance
to break builds of external plugins, considering how many internal ones it broke.
Most of these changes are either cmake-related or plugin-related.
Added a new test "plugins/legacy.zeek" to test that legacy Bro plugins
still work.
Also added a symlink bro-path-dev.in because some legacy Bro packages
won't install without it.
