Enable page allocator poisoning in zIPL

Description

To enable poisoning of free pages, check that all boot entries in /boot/loader/entries/*.conf have page_poison=1 included in its options.
To ensure that new kernels and boot entries continue to enable page poisoning, add page_poison=1 to /etc/kernel/cmdline.

Rationale

Poisoning writes an arbitrary value to freed pages, so any modification or reference to that page after being freed or before being initialized will be detected and prevented. This prevents many types of use-after-free vulnerabilities at little performance cost. Also prevents leak of data and detection of corrupted memory.