mktempd: use GNU-style -t if available This better satisfies Jim Meyering’s point in: https://lists.gnu.org/archive/html/bug-gnulib/2016-03/msg00074.html that it’s more useful when each temporary test directory name includes the corresponding test’s name. * build-aux/mktempd: Try mktemp -t only with GNU syntax, so that NetBSD mktemp fails. Also, reject templates beginning with "-" so that they are not treated as options. * tests/init.sh (mktempd_): Likewise.