Obsolete p_secstodate.

This patch, relative to a tree with
<https://sourceware.org/ml/libc-alpha/2017-11/msg00797.html> (pending
review) applied, obsoletes p_secstodate, making the underlying
function __p_secstodate into a compat symbol not available for new
binaries or ports.  The calls in ns_print.c (part of incomplete
handling of TKEY) are changed to use %lu to print times instead of
trying to pretty-print the times any more.

Tested for x86_64.

	* resolv/res_debug.c (p_secstodate): Condition definition on
	[SHLIB_COMPAT (libresolv, GLIBC_2_0, GLIBC_2_27)].  Define
	directly as __p_secstodate, and as a compat symbol.  Do not use
	libresolv_hidden_def.
	* resolv/resolv.h (p_secstodate): Remove macro and function
	declaration.
	* resolv/ns_print.c (ns_sprintrrf): Print times with %lu, not
	using p_secstodate.
	* include/resolv.h (__p_secstodate): Do not use
	libresolv_hidden_proto.
	* resolv/Makefile (tests): Move tst-p_secstodate to ....
	(tests-internal): ... here.
	* resolv/tst-p_secstodate.c: Include <shlib-compat.h>.  Condition
	all contents on [TEST_COMPAT (libresolv, GLIBC_2_0, GLIBC_2_27)]
	and declare and use __p_secstodate and use compat_symbol_reference
	in that case.
	[!TEST_COMPAT (libresolv, GLIBC_2_0, GLIBC_2_27)] (do_test): Add
	implementation returning 77.