freebsd_network: Remove the need for <kernel_c++_structs.h>.

Use sizeof()+roundup() to build a char[] of the correct size,
and add a static_assert to ensure it stays so.

The <kernel_c++_structs.h> header needs to have its dependency
manually declared, and not all consumers of this file properly
declared it as such. This then fixes a concurrent build problem.

Fixes #17965 as this was the only consumer of this header.

freebsd_network: Implement cv_destroy.

freebsd_network: Rewrite ConditionVariable-using code.

The old code was not at all correct with respect to interlocking.
The new code should be, and is also much simpler.