iscntrl behavior does not agree with the documentation

[Steven Bellovin]

Index: 	lib/libc/gen/ctype_.c

Description:
	The man page says that iscntrl() returns true for characters less
	than 040.  It doesn't for ^I, ^J, ^K, ^L, ^M.
Repeat-By:
	Look at the file...
Fix:
	Change the _S to _S|_C for those entries.  *Don't* simply change
	the man page; it agrees with the documentation and behavior of
	System V and 8th Edition.