Clarify posix typedef declarations.

Cython/Includes/posix/types.pxd

+# Note that the actual size of these types is system-dependant, and
+# can't be detected at C compile time.  However, the generated C code
+# will correctly use the actual size of these types *except* for
+# determining promotion in binary arithmetic expressions involving
+# mixed types.  In this case, operands are promoted to the declared
+# larger type, with a bias towards typedef types.  Thus, with the
+# declarations below, long + time_t will result in a time_t whereas
+# long long + time_t will result in a long long which should be
+# acceptable for either 32-bit or 64-bit signed time_t (though admittedly
+# the POSIX standard doesn't even specify that time_t must be an integral
+# type).
+
 cdef extern from "sys/types.h":
     ctypedef long blkcnt_t
     ctypedef long blksize_t
@@ -5,7 +17,7 @@ cdef extern from "sys/types.h":
     ctypedef long dev_t
     ctypedef long gid_t
     ctypedef long id_t
-    ctypedef long ino_t
+    ctypedef unsigned long ino_t
     ctypedef long mode_t
     ctypedef long nlink_t
     ctypedef long off_t