introduce a couple of new turn-off-gcc-warning variables and use them.

GCC 14 has a new annoying calloc() checker that we turn off in a bunch
of places, and there are a few more dangling-pointer issuse that come up,
but seem bogus.

Centralize the base rump libraries into a variable used by all the other
Makefiles so that we can make changes to it centrally as needed and have
less mess. Fixes the sun2 build that needs rumpvfs after librump after
the latest changes.

branches: 1.5.24;
Add bpf/t_mbuf test to the build.

Fix static linking for the tests: -lrump is also used by -lrumpuser,
so we also need -lrump after -lrumpuser. Fixes build for sun2.

branches: 1.3.12; 1.3.22;
PR/44310: Alexander Nasonov: write to /dev/bpf truncates size_t to int

Add an xfail test for the mbuf leak described in PR kern/44196.

This is yet another example of a simple test which would be much
trickier to execute against the host kernel. You would either need
to put networking in a complete lockdown, or do some "statistical"
methods where you trigger the bug many many times and attempt to
ascertain a rising trend in mbuf count. And, of course, the leaked
mbufs don't go away from the host kernel once the test ends. In
contrast, we *know* that there is no other networking activity in
a rump kernel, so we can execute the operation exactly once, plus
the leaked mbuf "disappears" when the test is done.

Check that bpf doesn't accept programs with divide-by-zero in them.
Example filter from Guy Harris via PR kern/43185.