Squashed commit of the following:
Correct some additional compile-related issues.
Move include/nuttx/binfmt/builtin.h to include/nuttx/lib/builtin.h. Move apps/builtin/lib_builtin_forindex.c to libs/libc/builtin/lib_builtin_forindex.c.
Move binfmt/libbuiltin to libs/libc/builtin. There are calls made directly from apps/nshlib into this logic and hence, must be part of a library that can be shared between the OS and applications.
many locations: Change occurences of open() followed by file_detach() to file_open(). Change most non-controversion calls to open() to nx_open().
fs/inode/fs_fileopen.c: Flesh out file_open() with some interim, placeholder logic.
fs/inode/fs_fileopen.c: Add a framework for a file_open() implementation (no real logic in place yet).
fs/vfs/fs_open.c: Add nx_open() which is the same as open() except that it does not create a cancellation point nor does it modify the errno variable.
