devtool: check locale and refuse to start if it isn't UTF-8

devtool: check locale and refuse to start if it isn't UTF-8

We need to ensure the locale is UTF-8 or otherwise strange errors will
occur later on during execution - the same reason we check this in
BitBake itself. Unfortunately this check has to be before command line
parsing and therefore showing the help text in response to --help, since
that relies upon parsing bitbake's configuration (as we need to load
plugins in other layers).

Fixes [YOCTO #10908].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>