Add Documentation/sysctl/ctl_unnumbered.txt

Poeple keep on adding new numbered sysctls, when they're supposed not to.

Add a documentation file which explain why new sysctls should use
CTL_UNNUMBERED. The next patch will sprinkle pointers to this throughout
sysctl.c.

Eric provided the text (thanks)

Cc: "Eric W. Biederman" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/sysctl/ctl_unnumbered.txt

···
+
+Except for a few extremely rare exceptions user space applications do not use
+the binary sysctl interface.  Instead everyone uses /proc/sys/...  with
+readable ascii names.
+
+Recently the kernel has started supporting setting the binary sysctl value to
+CTL_UNNUMBERED so we no longer need to assign a binary sysctl path to allow
+sysctls to show up in /proc/sys.
+
+Assigning binary sysctl numbers is an endless source of conflicts in sysctl.h,
+breaking of the user space ABI (because of those conflicts), and maintenance
+problems.  A complete pass through all of the sysctl users revealed multiple
+instances where the sysctl binary interface was broken and had gone undetected
+for years.
+
+So please do not add new binary sysctl numbers.  They are unneeded and
+problematic.
+
+If you really need a new binary sysctl number please first merge your sysctl
+into the kernel and then as a separate patch allocate a binary sysctl number.
+
+([e-mail], June 2007)