Implement uptime

uptime version for linux only. First I use strftime to format time but removed it because of simplicity and less dependencies.
It uses /proc/uptime for uptime, code from users for the number of users and getloadavg() for load average. Both /proc/uptime and getloadavg are unix only (linux only?) so in the meantime I added uptime to UNIX_PROGS.
Based roughly on the code from C coreutils.