procd: add full service shutdown prior to sysupgrade

procd: add full service shutdown prior to sysupgrade

Currently OpenWrt will use the kill loop in stage2 to stop tasks. This can
fail as seen with wpa_supplicant not properly shutting down with certain
mesh configurations. Trigger the existing service_stop() code path for all
services just before exec'ing to upgraded.

Signed-off-by: John Crispin <[email protected]>
[make use of *_safe function now introduced for that purpose, also stop
container instances]
Signed-off-by: Daniel Golle <[email protected]>