net: add mechanisms to force go or cgo lookup, and to debug default strategy
GODEBUG=netdns=1 prints a one-time strategy decision. (cgo or go DNS lookups)
GODEBUG=netdns=2 prints the per-lookup strategy as a function of the hostname.
The new "netcgo" build tag forces cgo DNS lookups.
GODEBUG=netdns=go (or existing build tag "netgo") forces Go DNS resolution.
GODEBUG=netdns=cgo (or new build tag "netcgo") forces libc DNS resolution.
Options can be combined with e.g. GODEBUG=netdns=go+1 or GODEBUG=netdns=2+cgo.