Rely on -user for lfe shell

Use -user lfe_boot instead of -noshell and -noinput.

This allows standard input functions to still work with
lfe since input is still available. -noshell and -noinput
now can also be given by the developer in case he
wants to disable one or the other.

Besides, not using `-user` caused us a bunch of troubles
in Elixir. Since I used the lfe shell as basis to build elixir's
shell, I thought we should contribute our findings back.

This option works fine since R15, I did not check previous
versions. :)