. new kernel call sysctl for generic unprivileged system operations;

now used for printing diagnostic messages through the kernel message
   buffer. this lets processes print diagnostics without sending messages
   to tty and log directly, simplifying the message protocol a lot and
   reducing difficulties with deadlocks and other situations in which
   diagnostics are blackholed (e.g. grants don't work). this makes
   DIAGNOSTICS(_S), ASYN_DIAGNOSTICS and DIAG_REPL obsolete, although tty
   and log still accept the codes for 'old' binaries. This also simplifies
   diagnostics in several servers and drivers - only tty needs its own
   kputc() now.
 . simplifications in vfs, and some effort to get the vnode references
   right (consistent) even during shutdown. m_mounted_on is now NULL
   for root filesystems (!) (the original and new root), a less awkward
   special case than 'm_mounted_on == m_root_node'. root now has exactly
   one reference, to root, if no files are open, just like all other
   filesystems. m_driver_e is unused.

kernel/system.c

@@ -241,6 +241,7 @@ PRIVATE void initialize(void)
   /* System control. */
   map(SYS_ABORT, do_abort);		/* abort MINIX */
   map(SYS_GETINFO, do_getinfo); 	/* request system information */ 
+  map(SYS_SYSCTL, do_sysctl); 		/* misc system manipulation */ 
 
   /* Profiling. */
   map(SYS_SPROF, do_sprofile);         /* start/stop statistical profiling */