mirroring: always set a UTF-8 locale for fast-import providers

mirroring: always set a UTF-8 locale for fast-import providers

When running the various fast-import providers, some of them
*cough* python *cough* will mangle the output of non \x00 - \x7f
characters if the locale is not set to a UTF-8 one.

Unfortunately there is no universal UTF-8 locale (such as
"POSIX.UTF-8").  Attempting to use "POSIX.UTF-8" frequently
winds up being treated as "POSIX" which causes character
mangling.

Instead we use "en_US.UTF-8" because that should be understood
everywhere to mean UTF-8.  It's only set while the fast-import
providers *cough* python *cough* are being run and the value
that's used may be changed by altering the setting of the
use_locale= variable in shlib.sh.

For the rest of the time the scripts run, the default system
locale should be in effect to cause minimal disruption to any
translated messages.