\L and \U now persist until a \e or \E, despite an intervening \u or \l

The existing behavior would cause s/fOo/\L\u&/ to change fOo to FOo,
instead of the expected Foo.  The \l immediately overrode the effect of
the \U.

The one-off (\u/\l) and persistent (\U/\L) flags are now tracked
distinctly so the persistent ones are only permanently disabled by
another persistent flag or an \E.