UB in QProcessPrivate::startDetached()

Following commit c5c7712910f5de454ee0d9a30b9a848d023bb543, CodeChecker complains (report-ID: 1330341), about the code run in the child process immediately after the doFork() call, including a signal() call, that

This function call is prohibited after a successful vfork

Indeed, we're in the child process here and the vfork man-page says, explicitly:

(From POSIX.1) The vfork() function has the same effect as
fork(2), except that the behavior is undefined if the process
created by vfork() either modifies any data other than a variable
of type pid_t used to store the return value from vfork(), or
returns from the function in which vfork() was called, or calls
any other function before successfully calling _exit(2) or one of
the exec(3) family of functions.

It is possible this UB is more nominal than actual, but the code should at least contain a justification for ignoring the fact that it's UB.