dlopen, &c., and threads

[Taylor R Campbell <campbell%mumble.net@localhost>]

I see nothing in src/libexec/ld.elf_so synchronizing concurrent uses
of the dlopen family of functions in multiple threads.  The SUS does
not require them to be thread-safe, but is it intended, or documented
anywhere, that these are not in NetBSD; or am I missing something
about the implementation?