fpending, obstack, strerror-override: use pure+const function attrs

fpending, obstack, strerror-override: use pure+const function attrs

* lib/fpending.h (__fpending): Declare with the "pure" attribute.
* lib/obstack.c (_obstack_allocated_p): Likewise.
* lib/obstack.h (_obstack_memory_used): Likewise.
(_obstack_memory_used): Likewise.
* lib/strerror-override.h (strerror_override): Declare with
the "const" attribute.