[analyzer] Split unix.API up to UnixAPIMisuseChecker and UnixAPIPortabilityChecker

The actual implementation of unix.API features a dual-checker: two checkers in
one, even though they don't even interact at all. Split them up, as this is a
problem for establishing dependencies.

I added no new code at all, just merely moved it around.

Since the plist files change (and that's a benefit!) this patch isn't NFC.

Differential Revision: https://reviews.llvm.org/D55425

llvm-svn: 352278