realmd: Use UTF-8 locale when running with C
realmd: Use UTF-8 locale when running with C

It can happen that cockpit-bridge runs under the C locale, in which case
joining an IPA domain fails with

    ipa: ERROR: System encoding must be UTF-8, 'ANSI_X3.4-1968' is not supported. Set LC_ALL="C.UTF-8", or LC_ALL="" and LC_CTYPE="C.UTF-8"

This is actually bad advice on RHEL/CentOS 7, where C.UTF-8 still does
not exist.

Avoid this situation by explicitly setting C.UTF-8, or en_US.UTF-8 as a
fallback, when the locale is C.

Closes #9637

mvollmer/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,andreasn/cockpit,cockpituous/cockpit,garrett/cockpit,mvollmer/cockpit,cockpituous/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,andreasn/cockpit,garrett/cockpit,andreasn/cockpit,garrett/cockpit,garrett/cockpit,garrett/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,mareklibra/cockpit,cockpit-project/cockpit,mareklibra/cockpit,martinpitt/cockpit,cockpituous/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mareklibra/cockpit,mvollmer/cockpit,deryni/cockpit,andreasn/cockpit,martinpitt/cockpit,mareklibra/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit