Protect malloc, realloc and free calls with INT{ON,OFF} directly in chkalloc,

Protect malloc, realloc and free calls with INT{ON,OFF} directly in chkalloc,
ckrealloc and ckfree (added), respectively.  sh jumps out of the signal handler
using longjmp which is obviously a bad idea during malloc calls.

Note: I think there is still a small race here because volatile sig_atomic_t
only guarantees atomic reads and writes while we're doing increments and
decrements.

Protect a setmode call with INT{ON,OFF} as it calls malloc internally.

PR:		45478
Patch from:	Nate Eldredge