calendar, prettydate: Fix strftime encoding bug

strftime is a C function, it does not return decoded utf8.
Several places in ikiwiki manually decoded it, but at least two
forgot to.

Also, strftime might not return even encoded utf8, if LC_TIME is set
to a non-utf8 value. Went ahead and supported decoding whatever encoding
it uses.

The remaining direct calls to strftime() are all ones that first set
LC_TIME=C, in order to get times that are not for human display.