resolv: Move dn_comp to its own file and into libc

And reformat it to GNU style.

dn_comp is used in various programs, so keep it as a non-deprecated
symbol.  Switch to dn_comp (not __dn_comp) for the ABI name.  There
are no internal users, so interposition is not a problem.

The __dn_comp symbol was moved with scripts/move-symbol-to-libc.py
--no-new-version.  dn_comp@@GLIBC_2.34 was added with
make update-all-abi.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>