Makes ssl-expiry-date compatible with BSD tools (mktemp and make)

On BSD flavored UNIXes mktemp and date behave differently, this pull request solves that.

* on BSD's mktemp a prefix or template is mandatory while n GNU's mktemp its not and the template defaults to tmp 
* on BSD's date the --date parameter doesn't exist but it uses -v instead for that purpose (with a more complicated syntax)