Fix Sparse warnings:
 * use NULL instead of plain 0
 * rearrange header include order to avoid redefining types accidentally
 * ANSIfy SLIRP
 * avoid "restrict" keyword
 * add static



git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6736 c046a42c-6fe2-441c-8c8c-71466251a162