Make ld-elf.so.1 directly executable.

Check if passed phdr is actually phdr of the interpreter itself, and
decide that this is the case of direct execution. In this case, the
binary to activate is specified in the argv[1]. After opening it,
shift down on-stack structure with argv, env and aux vectors to
emulate execution of the binary and not of the interpreter.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D10701