problems noted by fgsch@ and deraadt@

original commit was done?)

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@

ok henning@

ok henning@

pointed out by todd@

ok, deraadt@

ok miod@

As requested by deraadt@

ok millert@

generic as /dev/smartcard or similar.
deraadt@ ok.

make -> ${MAKE}
add phony targets
remove unnecessary subshells
install kernel in a separate target from building.
some SUDO.
okay deraadt@
(checked bootstrap on i386, should be safe elsewhere, and trivial to fix
anyways)

ok deraadt@

crashes sh, EVEN after that possible fix.  It appears to overflow the
ramdisk _before_ it dumps core.  Noone was shown these changes.  They were
just tossed in.  This is not a development process that will work, so I
am just backing out all the changes in the last 24 hours.