scripts/config: use sed's POSIX interface

Script `config' relies on extensions of `GNU sed', and is thus not
working on all Unixes:
  - in-place edition of files (-i), which can be replaced with
    a temporary file;
  - extended-regexps (-r), which can be split into basic regexps;
  - single-line calls to `a' command, while some implementations
    require a leading newline before the parameter.

Rewrite calls to `sed' to comply with POSIX interface, and move them
to helper functions.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>