BUG/MEDIUM: mworker: don't fill the -sf argument with -1 during the reexec

BUG/MEDIUM: mworker: don't fill the -sf argument with -1 during the reexec

Upon a reexec_on_failure, if the process tried to exit after the
initialization of the process structure but before it was filled with a
PID, the PID in the mworker_proc structure is set to -1.

In this particular case the -sf argument is filled with -1 and haproxy
will exit with the usage message because of that argument.

Should be backported in 2.0.