Avoid pass-by-reference in C-style ABI of libnetd_resolv

This is in preparation to enabling the ABI checker on libnetd_resolv.

Switching resolv_init()'s argument from reference to pointer is
technically an ABI change, but in practice most calling conventions
pass pointers and references exactly the same way (I verified only
aarch64).

Test: as follows:
  - build libnetd_resolv
  - install on system with old netd binary
  - reboot
  - atest resolv_integration_test

Change-Id: Iebf385036010b4065afe74209a9f7cf021ca5a20