selinux-h: improve stub types and add more stub functions

selinux-h: improve stub types and add more stub functions

* lib/se-selinux.in.h: Change security_context_t to a typedef
rather than a define, as it's a pointer type and so is better
as a typedef to avoid issues declaring multiple variables
with the comma operator.  Also add stub for string_to_security_class().
* lib/se-context.in.h: Add stub functions for
context_{type,range,role,user}_get().