dlfcn-util: add static asserts ensuring our sym_xyz() func ptrs match the types from...

dlfcn-util: add static asserts ensuring our sym_xyz() func ptrs match the types from the official headers

Make sure that the sym_xyz function pointers have the types that the
functions we'll assign them have.

And of course, this found a number of incompatibilities right-away, in
particular in the bpf hookup.

(Doing this will trigger deprecation warnings from libbpf. I simply
turned them off locally now, since we are well aware of what we are
doing in that regard.)

There's one return type fix (bool → int), that actually matters I think,
as it might have created an incompatibility on some archs.