Add howmany() macro since some systems have this in sys/param.h

which we no longer include.

include/missing.h

@@ -149,6 +149,13 @@
 # define MAX(a,b) (((a)>(b))?(a):(b))
 #endif
 
+/*
+ * Some systems define this in <sys/param.h> but we don't include that anymore.
+ */
+#ifndef howmany
+# define howmany(x, y)	(((x) + ((y) - 1)) / (y))
+#endif
+
 /*
  * Older systems may be missing stddef.h and/or offsetof macro
  */