ipkg: add a default postinst/prerm script

the postinst script enables/starts the init.d scripts upon package installation
and installs the users required by the package.

the prerm script stops and disables the init.d scripts.

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42470 3c298f89-4303-0410-b956-a3cf2f4a3e73