mirroring: choose a better UTF-8 locale

mirroring: choose a better UTF-8 locale

Since 8331505e we set a UTF-8 locale so that some of the fast
import providers *cough* python *cough* won't unnecessarily
mangle characters from the source repository when generating
the fast-export stream.

For this to work, however, the chosen UTF-8 locale MUST be one
listed in the `locale -a` output.

Parse the `locale -a` output and choose a suitable UTF-8 locale
instead of assuming that there is any well-known UTF-8 locale
that will work everywhere.