supply template argument to mktemp

supply template argument to mktemp

mktemp versions before 1.5 did not allow omitting the template
(prefix.XXXXXX) argument.  Thus on Mac OS X,

	$ mktemp -d
	usage: mktemp [-d] [-q] [-t prefix] [-u] template ...
	       mktemp [-d] [-q] [-u] -t prefix

So supply a filename template.  To maintain the intended
behavior, we have to add -t, too.  It was implied before.

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>