musl libc does not support lower level libresolv API (_res)

[thommey]

musl does not support _res.nscount and _res.nslist at all - only the higher level API for sending out queries, which breaks dns.mod completely.

See https://git.musl-libc.org/cgit/musl/tree/src/network/res_state.c and related, and http://wiki.musl-libc.org/wiki/Functional_differences_from_glibc#Name_Resolver_.2F_DNS for their reasoning.
Also, http://lists.busybox.net/pipermail/busybox/2014-November/081960.html

(partially relevant:
gliderlabs/docker-alpine#8
http://www.openwall.com/lists/musl/2015/09/04/4
http://www.openwall.com/lists/musl/2014/04/30/7
http://www.openwall.com/lists/musl/2014/05/04/20
http://www.openwall.com/lists/musl/2014/06/01/2
)

I would suggest detecting _res being all 0 after res_init() (not just _res.nscount, that can be normal) at dns.mod's configure and disabling dns.mod in that case - it completely relies on libresolv for detecting nameservers, building the query packet, etc.

found by geo (for reference)