(at leat on solaris) strerror() itself may fail for an out-of-memory
condition (because the localization code contains memory
allocations). In order to handle this situation, we need to save the
original errno because strerror() may also set errno.

This issue exists in many more places all over the code, but in the
pool_breed case we likely failed pthread_create for an out-of-memory
condition, and in the panic handler we want to make sure that we
trip no follow-up panic under any circumstances.

In general, while fixing all strerror() calls would unnecessarily
complicate the code, doing so should be justified for these cases.

Fixes #2815