[Pem] freedesktop.org standard

[For anything and everything about GNU Pem]

Hi if we add this code just after line 246 in pem.pl we can achieve compliance.

if (defined ($ENV{"XDG_DATA_HOME"}) && $ENV{"XDG_DATA_HOME"} ne "")
{
    $pemdir = $ENV{"XDG_DATA_HOME"}."/pem";
}

This will place all data in the XDG_DATA_HOME folder defined in the variable.

If the variable does not exist or is null it will default to ~/.pem