*: introduce and use xfork()

*: introduce and use xfork()

function                                             old     new   delta
xfork                                                  -      20     +20
msh_main                                            1377    1380      +3
mod_process                                          455     446      -9
forkexit_or_rexec                                     30      17     -13
expand_variables                                    1434    1421     -13
open_transformer                                      91      76     -15
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(add/remove: 1/0 grow/shrink: 1/4 up/down: 23/-50)            Total: -27 bytes