So, the startup interface between ld.elf_so and executables is a
complete crock.

1) In order to allow init functions in shared libraries to make
   reference to the environment, __progname, et al, we currently have
   ld.elf_so inspect the executable's symbol table to find these items
   and patch them.  Among other things, this violates the separation
   between ld.elf_so and the executable's execution environment.  It
   is not acceptable.

2) We pass a pointer to an INTERNAL DATA STRUCTURE in ld.elf_so back
   to the executable, to be used by the dl*(3) functions to fetch
   function pointers.

What I propose is this:

a) Replace the Obj_Entry pointer used for __mainprog_obj with a
   pointer to a fictitious data structure which contains pointers to
   the dl*(3) entry points, _rtld_exit(), and a new function (herein
   temporarily called _rtld_do_init()).

b) Rather than running the init functions immediately, control is
   instead passed to the executable, which uses the function pointer
   to call _rtld_do_init(), after setting up __progname, __ps_strings,
   environ, etc.

c) Therefore, the above-mentioned symbols will not be hacked by
   ld.elf_so.

d) The `cleanup' function pointer will no longer be passed or used by
   crt0, and will instead be taken from the structure mentioned in
   part a.

e) To enable ld.elf_so to transparently detect new executables and
   switch modes of operation, and to prevent chicken-and-egg problems,
   the version number for the .note.netbsd.ident tag (in
   ld.elf_so/sysident.h) will for a time be set to a static value.  It
   may be permanently left as a static value in sysident.h, but I'm
   not certain of that yet.

f) Compatibility code in ld.elf_so will be present under `#ifdef
   COMPAT_16', which will be enabled by default.

This will happen next Wednesday unless someone sways me with a sound
technical reason to not do it.