livecdfs-update.sh: Escape ampersands in STARTX sed expression

livecdfs-update.sh: Escape ampersands in STARTX 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

when we want:

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

with ${first_user} expanded in both cases.

Reviewed-by: Matt Turner <mattst88@gmail.com>