Make resolv_gethostbyname() re-entrant

The new call signature is closer to gethostbyname2_r() and no longer
relies on thread-local storage for its buffers. res_get_static() is now
down to a single caller.

Test: m, flash, atest
Change-Id: Ic6e2c0292fece31c2586a81d8fb6a8df20c4f9f5