selftests/exec: test interpreters bound to a 'B' entry

One handler, one entry registered disabled, an interpreter per guest
architecture bound to a file one write at a time. The load program picks
one by name per exec:

- an aarch64 binary runs the interpreter bound as "first" and a riscv one
  the interpreter bound as "second", from a single entry and a single
  handler

- unlinking a bound interpreter and putting a different binary in its
  place changes nothing, which is what the binding exists for

- the entry reports what it bound, under the names it bound them as

- a name the entry did not bind fails the exec with -ENOENT rather than
  falling back to anything

- activating the entry refuses further binding with -EBUSY, a later
  disable does not undo that, and an entry registered without 'D' never
  accepted a '+' write to begin with

- a name binds one interpreter, and control characters are refused

- the command has to end at the write, bytes past an embedded nul are
  refused

- an entry binds at most 100 interpreters, the next one is refused with
  -ENOSPC

The test interpreter prints its argv[0], which is the path the kernel ran
that copy under, so one binary installed at two paths tells the harness
which of them the program picked.

Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-8-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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