mpstat, iostat: Use decimal point for FP values in JSON output

mpstat, iostat: Use decimal point for FP values in JSON output

This patch sets LC_NUMERIC variable to "C" to make sure JSON output (as
displayed by mpstat and iostat commads) is locale-independent and
floating-point values use a decimal point.
This should make JSON output compliant with RFC7159.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>