rtld: add LD_NO_DL_ITERATE_PHDR_AFTER_FORK env var

which makes threaded fork ignore the phdr rtld lock, in particular
allowing the dl_iterate_phdr() to block in callback. The cost is that
the image started in this mode cannot use dl_iterate_phdr() after fork.

PR: 280318
Sponsored by: The FreeBSD Foundation
MFC after: 1 week