sh: Assert INTOFF rather than applying it in ck*

[jilles]

Summary

As I noted in https://reviews.freebsd.org/D22756, INTOFF should be in
effect when calling ckmalloc/ckrealloc/ckfree to avoid memory leaks and
double frees. Therefore, change the functions to check if INTOFF is in
effect instead of applying it.

Test Plan

installworld and kyua test
code inspection of poudriere custom builtins