[analyzer] Turn on by default the Malloc Checker and a couple of CString

checks:

- unix.Malloc - Checks for memory leaks, double free, use-after-free.
- unix.cstring.NullArg - Checks for null pointers passed as arguments to
CString functions + evaluates CString functions.
- unix.cstring.BadSizeArg - Checks for common anti-patterns in
strncat size argument.

llvm-svn: 150988