CHOP-753: Add classic-C-style diagnostics around popen("vm_stat"). On Mac, where LLMemoryInfo relies on a child process rather than any sort of internal system API, try to produce more informative LL_WARNS output if popen() fails to run vm_stat, or if vm_stat terminates with nonzero rc.
CHOP-753: Add classic-C-style diagnostics around popen("vm_stat").
On Mac, where LLMemoryInfo relies on a child process rather than any sort of
internal system API, try to produce more informative LL_WARNS output if
popen() fails to run vm_stat, or if vm_stat terminates with nonzero rc.

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