Use strerror_r(3) for the printf family %m.

There's TLS space used for unknown errno values, and a call to printf
shouldn't clobber that. No-one will ever hit this in real life, but
since it's easily fixed...

Bug: http://b/112776560
Test: ran tests
Change-Id: I8c2437f2e5214e652119791d4e162a197b049d5b