runtest: better command line API * t/ax/runtest.in: Accept options '-k' and '--keep-testdirs' (same as exporting '$keep_testdirs' to "yes"). To improve compatibility with the "make check" interface, allow environment variables to be passes on the command line. Minor adjustments while at it. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>