Move a few internal constants out of resolv_params.h

This is a preliminary step to cleanly separate the public headers
from the internal ones. No funtionality changes.

In the process, I also un-ifdefd RES_USE_EDNS0 and EDNS0_PADDING, since
they were always defined.

For now I'm leaving alone these #ifdefs, which need some more thought:

 - HAVE_SIN6_SCOPE_ID: currently disabled, but <linux/in6.h> has it
 - HAS_INET6_STRUCTS: currently disabled, will need test coverage?
 - CANNOT_CONNECT_DGRAM: currently disabled

Test: build, flash, then 'atest netd_integration_test'
Change-Id: I4cee37d484a5183a2db082d502b2badcaa83e7e4