sysinfo.pl 1.2.3: fix memory used/free swapped in display and percent

[TehPeGaSuS]

memoryusage() computes $vard as the actual used memory (Total - Free - Buffers - Cached), but the final return statement treats it as if it were free memory instead:

$varp = sprintf("%.2f", 100-($vard / ($vara-$vard) * 100));
return human_size($vara-$vard)."/".human_size($vara)." ($varp%)";

This mislabels free memory as used in the displayed output, and can produce a negative percentage whenever used memory exceeds free memory, e.g.:

Memory Usage: 2.83GB/7.57GB (-67.52%)

(on a host where 4.74GB was actually in use, i.e. ~62.6%).

Fix by using $vard directly for both the display and the percentage:

$varp = sprintf("%.2f", ($vard / $vara) * 100);
return human_size($vard)."/".human_size($vara)." ($varp%)";

This follows up #610 (merged as 1.2.2), which fixed the kernel-version detection but left this separate used/free swap in place.