Break inclusion cycle: prototypes.h and commands.h included each

other.  Ensure headers in include/ can be included in any order
(except for econfig-spec.h, which is special).  New header types.h to
help avoid inclusion cycles.  Sort include directives.  Remove some
superflous includes.

src/lib/lwp/sel.c

@@ -34,9 +34,7 @@
 
 #include <config.h>
 
-#include <stdlib.h>
 #include <errno.h>
-#include <sys/types.h>
 #include <sys/uio.h>
 #include <sys/file.h>
 #include <sys/time.h>