These functions failed with "unknown system failure" whenever strftime()
returned 0 for a non-empty format, but 0 also means the format expands to
an empty string, as %p does in a locale without AM/PM.
Prepend a character to the format and trim it from the result, so that 0
can only mean the result is too long. Now 0 | strflocaltime("%p") gives
"" in fr_FR.UTF-8, and a result that is still too long fails with
"result too long" error instead. The size limit is unchanged.
