[MDC-826] simetbox-openwrt-simet-ma: simet-ma_run rpcd

[MDC-826] simetbox-openwrt-simet-ma: simet-ma_run rpcd

Add a new field to rpcd simet_ma simet-engine-status:
"main_measurement_running":<boolean>, which is true when the main
measurement run (twamp + tcpbw + traceroute + geo) is active.

It does not cover auxiliary measurements that are not members of the
main measurement schedule / simet-runner.

Add a new rpcd method, simet_ma start_measurement_run, which triggers a
simet-ma_run.sh execution in background.  It reports a status of
"already running", "started", or "failed".

It requires simet-ma 0.16.6 or later.