* lib/install-sh (posix_mkdir): Reject FreeBSD 6.1 mkdir -p -m,

* lib/install-sh (posix_mkdir): Reject FreeBSD 6.1 mkdir -p -m,
which incorrectly sets the mode of an existing destination
directory.  In some cases the unpatched install-sh could do the
equivalent of "chmod 777 /" or "chmod 0 /" on a buggy FreeBSD
system.  We hope this is rare in practice, but it's clearly worth
fixing.  Problem reported by Alex Unleashed in
<http://lists.gnu.org/archive/html/bug-autoconf/2006-10/msg00012.html>.
Also, don't bother to check for -m bugs unless we're using -m;
suggested by Stepan Kasal.