resolve: dedup entries in /etc/hosts

resolve: dedup entries in /etc/hosts

This improves the performance of parsing the file and reduces memory pressure.

Running 'fuzz-etc-hosts timeout-strv' with valgrind,

Before:
total heap usage: 321,020 allocs, 321,020 frees, 15,820,387,193 bytes allocated
real    0m23.531s
user    0m21.458s
sys     0m1.961s

After:
total heap usage: 112,408 allocs, 112,408 frees, 7,297,480 bytes allocated
real    0m8.664s
user    0m8.545s
sys     0m0.065s

Hopefully fixes oss-fuzz#47708 (https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47708).