libstand modification

https://phabric.freebsd.org/D443

the 64bit version of userboot has been screaming about bit shifting
operators for a while now.

The short explanation, amd64 sizeof(long) != i386 sizeof(long).

The long explanation is in the phabric diff and comments.

I see no reason to not commit this, but thought I'd throw it here for
your commentary.

sean