Have builtins use ck* malloc functions.

[bdrewery]

Summary

This will ensure interrupts are disabled and out-of-space is
handled gracefully.

[jilles]

Adding INTOFF/INTON around malloc/free prevents unsafety due to interrupting the allocator, but does not prevent memory leaks or double frees due to the administration getting out of sync with the allocator (for example, if the INTON in ckmalloc() notices an interrupt, the allocated memory will leak). It would make more sense if ckmalloc/ckrealloc/ckfree merely asserted that INTON is in effect. I experimented with that earlier but did not follow through.

Also, the exception thrown by ckmalloc and ckrealloc if they fail is unlikely to be handled properly by code not specially designed for it.

What I have done in usr.bin/printf is to use normal strdup and check its return value, and to add INTOFF/INTON around most of the program under #ifdef SHELL. (This has negative effects if writing to standard output blocks.)

[bdrewery]

Thanks for looking.