libstand's qdivrem.c assumes that sizeof(int) == sizeof(long), this is not
true on amd64 I'm not quite positive this is the "correct" solution for
this but it does seem to compile and shut up the spew of warnings when
compiling libstand for userboot.

Add two _Static_asserts() so that in the future somebody will get a compile
failure if an architecture develops that violates the assumptions of this
code. (strongly suggested by jmg)

Change commetns to indicate int types instead of long.  (noted by ian in
phabric review)

Phabric:    https://phabric.freebsd.org/D443