POSIX defines the minimum total size of argv + envp to ARG_MAX,
and leaves the inclusion of NULL terminators, pointers and / or
alignment bytes as implementation defined. The policy we implement is:

- Permit the total length to exceed ARG_MAX as long as it's not
  greater than 1/4 of the stack size. This is essentially the same
  policy as linux (see execve(2)), with the difference that we
  don't apply the MAX_ARG_STRLEN floor of 32 pages.

- Exclude auxv, NULL, andlignment bytes from the checked size, as
  these are of fixed size and occupy an insignificant portion of
  the stack.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>

Add binfmt loader for ELF. This is the default loader used on
execve().

Signed-off-by: Michalis Pappas <michalis@unikraft.io>