[LEDE-DEV] procd.sh: use parrameterized respawn values

[Claudiu Brasovean]

Continue the work started here: http://patchwork.ozlabs.org/patch/520859/
Extend /etc/config/system with parameters to set the default 
threshold and timeout values for procd launched services that
have respawn enabled.

	config service
		option respawn_threshold 3600
		option respawn_timeout 5
		option respawn_retry -1

This allows for cleaner init scripts, with system wide default values,
while allowing per service customisations