firewall-util: probe firewall backend in fw_ctx_new()

firewall-util: probe firewall backend in fw_ctx_new()

FirewallContext is used by networkd and nspawn. Both allocates the
context when it is really necessary. Hence, it is not necessary to delay
probing backend.

Moreover, if iptables backend is not enabled on build, and nftables is
not supported by kernel, previously `fw_nftables_init()` is called
everytime when we try to configure masquerade or dnat. It causes
significant performance loss.

Fixes test-firewall-util issue in #19052.