livecdfs-update.sh: Use `bash --login` to spawn startx

livecdfs-update.sh: Use `bash --login` to spawn startx

Starting a "login" version of Bash via `su` is tricky.  The naive:

  su - ${first_user} -c startx

fails because `su - ...` clears a number of environment variables (so
the prefixed `source /etc/profile` doesn't accomplish anything), but
Bash isn't started with the `--login` option, so it doesn't source
/etc/profile internally.  From bash(1):

  A login shell is one whose first character of argument zero is a -,
  or one started with the --login option.
  ...
  An interactive shell is one started without non-option arguments and
  without the -c option whose standard input and error are both
  connected to terminals (as determined by isatty(3)), or one started
  with the -i option...
  ...
  When bash is invoked as an interactive login shell, or as a
  non-interactive shell with the --login option, it first reads and
  executes commands from the file /etc/profile, if that file exists.
  After reading that file, it looks for ~/.bash_profile,
  ~/.bash_login, and ~/.profile, in that order, and reads and executes
  commands from the first one that exists and is readable.  The
  --noprofile option may be used when the shell is started to inhibit
  this behavior.

In order to get the login-style profile loading with a non-interactive
`su` invocation, you need to use something like:

  echo "${command}" | su - "${user}"

This starts a login shell and pipes the command in via stdin, which
seems to fake Bash into thinking its running from an interactive
terminal.  Not the most elegant, but the other implementations I can
think of are even worse:

  su - "${user}" -c "bash --login -c ${command}"
  su - "${user}" -c 'source /etc/profile &&
      (source .bash_profile || ...) && ${command}"

The old expression was broken anyway due to unescaped ampersands in
the sed expression.  From sed(1):

  s/regexp/replacement/
    Attempt to match regexp against the pattern space.  If successful,
    replace that portion matched with replacement.  The replacement
    may contain the special character & to refer to that portion of
    the pattern space which matched, and the special escapes \1
    through \9 to refer to the corresponding matching sub-expressions
    in the regexp.

This means that the old expression (with unescaped ampersands) lead
to:

  source /etc/profile ##STARTX##STARTX su - ${first_user} -c startx

with ${first_user} expanded.  This commented out startx, so it was
never run.