dladdr(3) is in POSIX-2024, though the structure type was renamed

dladdr(3) is in POSIX-2024, though the structure type was renamed
to match Solaris and _not_ match glibc/BSDs and no one caught it
before publication.  Provide the new name but keep providing the
existing names under the same conditions as before.  Update #include
visibility and dlfcn(3) manpage and add restrict qualifiers.

ok deraadt@