bpo-34523: Fix C locale coercion on FreeBSD CURRENT (GH-10672)

bpo-34523, bpo-35290: C locale coercion now resets the Python
internal "force ASCII" mode. This change fix the filesystem encoding
on FreeBSD CURRENT, which has a new "C.UTF-8" locale, when
the UTF-8 mode is disabled.

Add _Py_ResetForceASCII(): _Py_SetLocaleFromEnv() now calls it.