kernel: use 64-bit virtual entry point for expanded header, document behaviour in...

kernel: use 64-bit virtual entry point for expanded header, document behaviour in a.out(6)

For 64-bit architectures, the a.out header has the HDR_MAGIC flag set
in the magic and is expanded by 8 bytes containing the 64-bit virtual
address of the programs entry point. While Exec.entry contains physical
address for kernel images.

Our sysexec() would always use Exec.entry, even for 64-bit a.out binaries,
which worked because PADDR(entry) == entry for userspace pointers.

This change fixes it, having the kernel use the 64-bit entry point
and document the behaviour in the manpage.