Overhaul the manual, esp. the Autotest chapter.

Overhaul the manual, esp. the Autotest chapter.

* doc/autoconf.texi (Installation Directory Variables):
Replace some uses of @var with @code.
(Special Shell Variables): Fix misordered paragraph about IFS.
(Writing Testsuites): Include paragraph following AT_TESTED in
the macro definition.
(testsuite Invocation): Failed tests are not rerun.
(testsuite Scripts, Autotest Logs, testsuite Invocation)
(Making testsuite Scripts): Minor edits for consistency and
language.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>