resolver: abuse _res a little less

In the past, _res was a global variable. Now, it's multiple function calls.
Several functions in the resolver use _res multiple times and therefore
call the function(s) far more than necessary.

Fix those callers to store the result of _res in a local variable.

Add __noinline to the definition of res_init() to avoid the code bloat
that these changes would have otherwise incurred. Thanks to jilles
for noticing this.

Reviewed by: jilles
MFC after: 1 week
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D3887