Linux: update process uid on change

Always check if the user of a process changed, e.g. by using setuid(2).

Process.c

@@ -968,6 +968,7 @@ void Process_init(Process* this, const Settings* settings) {
    this->show = true;
    this->updated = false;
    this->cmdlineBasenameEnd = -1;
+   this->st_uid = (uid_t)-1;
 
    if (Process_getuid == (uid_t)-1) {
       Process_getuid = getuid();