Add a sysctl to option USER_LDT, to control its behaviour, which will be

disabled by default.
Enable with sysctl -w machdep.userldt=1
This will allow people to use the few ports that require this functionality
to work without requiring the user to recompile a kernel.
The option USER_LDT remains in order to not increase size on the
installation media. It is now enabled in GENERIC.
Per espie@'s idea, but my diff was much simpler than his; ok deraadt@

src/etc/etc.i386/sysctl.conf

@@ -2,3 +2,5 @@
 #machdep.apmwarn=10		# battery % when apm status messages enabled
 #machdep.apmhalt=0		# 1=powerdown hack, try if halt -p doesn't work
 #machdep.kbdreset=1		# permit console CTRL-ALT-DEL to do a nice halt
+#machdep.userldt=1		# allow userland programs to play with ldt,
+				# required by some ports