pptpd: Update init script

This replaces the use of uci_validate_section() with
uci_load_validate(), which removes the need to declare local variables
for every config option.

This also adds a service_triggers() function and removes some
unnecessary curly brackets.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

net/pptpd/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=pptpd
 PKG_VERSION:=1.4.0
-PKG_RELEASE:=3
+PKG_RELEASE:=4
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=@SF/poptop