package/base-files: enhance & cleanup service wrappers

package/base-files: enhance & cleanup service wrappers

 * add service_check function for checking if a process is alive
 * add service_signal for sending arbitrary signals to a process
 * change service_stop to send a TERM signal first, wait for the process to die and send a KILL signal if it doen't
 * have service_kill print a warning on STDERR stating it has been deprecated

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28865 3c298f89-4303-0410-b956-a3cf2f4a3e73