Avoid overflows and underflows in sscanf and friends

(Patch from teor on 13104)
Extra tests for tor_memeq and memcmp

(Patch from teor; part of 13104)
Avoid divide by zero and NaNs in scale_array_elements_to_u64

Patch from teor; part of 13104
Avoid an overflow on negation in format_helper_exit_status

Part of 13104; patch from teor.
Add a changes file for bug 13104
C90 compliance for #13104 fixes
Merge remote-tracking branch 'public/bug13104_025'