lib/, tests/: addsl(): Add addsl(), a variadic macro

lib/, tests/: addsl(): Add addsl(), a variadic macro

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>