- use uci_load_validate()
 - remove stop_service()
 - fix validation of "args" option

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

The can be convenient for running commands or services as procd services
without needing to separately write initscripts, just uci configuration.

The package was imported from [1].

 [1] 0a85f5c75f/pservice

Ref: https://github.com/yousong/waller/issues/1
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>