* doc/autoconf.texi (Writing Testsuites): Mention autotest
namespace.
(Writing Testsuites) <AT_INIT>: Mention mandatory macros.
(Making testsuite Scripts): Document AT_PACKAGE_URL.
* tests/Makefile.am (package.m4): Follow our own advice.
* lib/autotest/general.m4 (AT_INIT): Give the user a hint about
package.m4. Enhance --help output.
(_AT_COPYRIGHT_YEARS): New macro, to make copyright bump easier.