Rename apps/include/apps.h to builtin.h. Move parts of...

Rename apps/include/apps.h to builtin.h.  Move parts of apps/builtins/exec_builtin.c to binfmt/libbuiltin/libbuiltin_utils.c

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5524 42af7a65-404d-4744-a932-0658087f49c3