clean up libsubid headers

Move libsubid/api.h into libsubid/subid.h, and document the api in subid.h

Signed-off-by: Serge Hallyn <serge@hallyn.com>

src/free_subid_range.c

@@ -1,6 +1,6 @@
 #include <stdio.h>
 #include <unistd.h>
-#include "api.h"
+#include "subid.h"
 #include "stdlib.h"
 #include "prototypes.h"
 

src/get_subid_owners.c

@@ -1,5 +1,5 @@
 #include <stdio.h>
-#include "api.h"
+#include "subid.h"
 #include "stdlib.h"
 #include "prototypes.h"
 

src/list_subid_ranges.c

@@ -1,5 +1,5 @@
 #include <stdio.h>
-#include "api.h"
+#include "subid.h"
 #include "stdlib.h"
 #include "prototypes.h"
 

src/new_subid_range.c

@@ -1,6 +1,6 @@
 #include <stdio.h>
 #include <unistd.h>
-#include "api.h"
+#include "subid.h"
 #include "stdlib.h"
 #include "prototypes.h"