stdenv/setup.sh: fix breakage when shebang contains '\'

Some programs, e.g. guile-config, has a shebang that ends in '\':

  #!/usr/bin/guile-1.8 \
  -e main -s
  !#
  ;;;; guile-config --- utility for linking programs with Guile
  ;;;; Jim Blandy <jim@red-bean.com> --- September 1997

This currently breaks patchShebangs:

  $ read oldPath arg0 args <<< 'shebang \'; echo $?
  1
  $ echo $oldPath
  shebang
  $ echo $arg0

  $ echo $args

(And setup.sh/patchShebangs is run with 'set -e' so any command that
return non-zero aborts the build.)

Fix by telling 'read' to not interpret backslashes (with the -r flag):

  $ read -r oldPath arg0 args <<< 'shebang \'; echo $?
  0
  $ echo $oldPath
  shebang
  $ echo $arg0
  \
  $ echo $args

Also needed: escape the escape characters so that sed doesn't interpret
them.