Add a variadic macro addsl() that accepts an arbitrary number of addends, instead of having specific versions like addsl2() or addsl3(). It is internally implemented by the addslN() function, which itself calls addsl2(). addsl3() is now obsolete and thus removed. Code should just call addsl(). Link: <https://github.com/shadow-maint/shadow/pull/882#discussion_r1437155212> Cc: Serge Hallyn <serge@hallyn.com> Signed-off-by: Alejandro Colomar <alx@kernel.org>