netd: Use a persistent iptables[6]-restore process

iptables-restore and ip[6]tables-restore are forked on demand
whenever we need them, and their stdin/out/err are replaced by
pipes to the parent process. All commands are sent via the stdin
pipe. We also add SIGCHLD handling so that we can detect error
conditions and restart the process whenever required.

Bug: 32323979
Test: Manual
Test: netd_unit_test, netd_integration_test

Change-Id: Ia12ee01f8b45e5b8a699c27eea1b6b55d40f16b5