Teach the FreeBSD/beri boot to "auto-detect" whether argument 4 (a3) is a
memory size of pointer to a struct bootinfo * by looking at its value and
seeing whether it is pointer-like.  If a pointer, assume it's a bootinfo
and extract memsize from it instead; otherwise, use it as memsize directly.
This allows kernels to support bootinfo being passed by loader (and boot2)
while still supporting older Miniboot setups.

When the module metadata pointer is available from loader, use it in the
kernel.

In the BERI kernel boot code, extract 'boothowto' (which includes boot flags
such as '-s') and 'envp' from passed module data.  Booting to single-user
mode using boot flags now works.