bpf-firewall: attach with BPF_F_ALLOW_MULTI if kernel supports

bpf-firewall: attach with BPF_F_ALLOW_MULTI if kernel supports

Reduced version of [0].
Use BPF_F_ALLOW_MULTI attach flag for bpf-firewall if kernel supports
it.

Aside from addressing security issue in [0] attaching with 'multi'
allows further attaching of cgroup egress, ingress hooks specified by
BPFProgram=.

[0] https://github.com/systemd/systemd/pull/17495/commits/4e42210d40f96e185a55d43041dd6b962ea830dd

(cherry picked from commit a442ccb4ebdbc3a9ff9d4504eb9724092149fd42)
(cherry picked from commit 0af3810d4b1c8bb4f0683758f47e042e8cb76972)