Fix `configure --help=recursive' in unconfigured/read-only trees.

Fix `configure --help=recursive' in unconfigured/read-only trees.

* lib/m4sugar/m4sh.m4 (_AS_LN_S_PREPARE): Avoid errors when `.'
is not writable, use 'cp -p' in this case, in the hope that it
will not actually be needed.  Still try removing files, in case
of other write errors.
* lib/autoconf/general.m4 (_AC_INIT_SRCDIR): For ac_confdir,
use $as_myself, not $0.
(_AC_INIT_HELP): For --help=recursive, if the subdir does not
exist, try again in the the source tree.  This change assumes
that the subpackage configure script is capable of running
--help=recursive in the source tree.
* tests/torture.at (Configuring subdirectories, Deep Package):
Adjust tests to expose both issues, also try invocation as
`sh configure ...' and plain `configure ...' with PATH adjusted.
* NEWS, THANKS: Update.
Report by Hans Ulrich Niedermann.