sysinfo.pl: fix memory usage on kernel >= 6.x, auto-scale units

[TehPeGaSuS]

The kernel version detection regex only matched majors 3-5:

my $l3 = 1 if $linux && $osv =~ /^2\.7/  || $osv =~ /^[3-5]\./;

On kernel 6.x/7.x neither $l26 nor $l3 gets set, so memoryusage() falls
through to the old vmstat-style branch which reads garbage from $vara/$vard,
causing the uninitialized-value warnings and "Illegal division by zero" from
#581.

This patch:

• detects any Linux kernel major >= 3 instead of hardcoding 3-5
• adds a small human_size() helper and uses it for both memory and disk
  usage output, so results are auto-scaled (B/KB/MB/GB/TB) instead of always
  hardcoded to MB/GB

Tested on kernel 7.0.0, /sysinfo memory and /sysinfo disk both work as
expected.

Closes #581

[flashcode]

Thanks, merged!