Fix compiler warning about mktimes()

The first argument to mktimes() is treated as const, so declare it as
const explicitly. This fixes warnings about time_fmt.
Use sysinfo() instead of getloadavg()

Replace the local getloadavg() call with a sysinfo(&info) call, where
info is global. This will allow other functions to use the results from
sysinfo() later.
Add memusage() function

Report percent of RAM used.

Note: this must be after loadavg() in forder.