resolv: Move dn_skipname to its own file and into libc

And reformat it to GNU style.

dn_skipname is used outside glibc, so do not deprecate it,
and export it as dn_skipname (not __dn_skipname).  Due to internal
users, provide a __libc_dn_skipname alias, and keep __dn_skipname
as a pure compatibility symbol.

__dn_skipname@GLIBC_2.0 was moved using the script, and
dn_skipname@@GLIBC_2.34 was added using make update-all-abi.

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