exec: Generic execfd support

Most of the support for passing the file descriptor of an executable
to an interpreter already lives in the generic code and in binfmt_elf.
Rework the fields in binfmt_elf that deal with executable file
descriptor passing to make executable file descriptor passing a first
class concept.

Move the fd_install from binfmt_misc into begin_new_exec after the new
creds have been installed.  This means that accessing the file through
/proc/<pid>/fd/N is able to see the creds for the new executable
before allowing access to the new executables files.

Performing the install of the executables file descriptor after
the point of no return also means that nothing special needs to
be done on error.  The exiting of the process will close all
of it's open files.

Move the would_dump from binfmt_misc into begin_new_exec right
after would_dump is called on the bprm->file.  This makes it
obvious this case exists and that no nesting of bprm->file is
currently supported.

In binfmt_misc the movement of fd_install into generic code means
that it's special error exit path is no longer needed.

Link: https://lkml.kernel.org/r/87y2poyd91.fsf_-_@x220.int.ebiederm.org


Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>