*: introduce and use xfork() and xvfork()

*: introduce and use xfork() and xvfork()

function                                             old     new   delta
launch_helper                                        170     169      -1
setup_heredoc                                        312     302     -10
handle_dir_common                                    367     354     -13
expand_vars_to_list                                 2456    2443     -13
open_transformer                                      89      74     -15
data_extract_to_command                              439     423     -16
do_ipaddr                                           1406    1389     -17
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/7 up/down: 0/-85)             Total: -85 bytes

Signed-off-by: Pascal Bellard <pascal.bellard@ads-lu.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>