BUG/MINOR: mworker: does not add the -sf in wait mode

BUG/MINOR: mworker: does not add the -sf in wait mode

Since the wait mode is automatically executed after charging the
configuration, -sf was shown in argv[] with the previous PID, which is
normal, but also the current one. This is only a visual problem when
listing the processes, because -sf does not do anything in wait mode.

Fix the issue by removing the whole "-sf" part in wait mode, but the
executed command can be seen in the argv[] of the latest worker forked.

Must be backported in 2.5.

(cherry picked from commit befab9ee4aa2dfe0405d7f1416991175be2fbe67)
[wla: in 2.5 we still need the -x in wait mode, because we don't know
anymore the stats socket in the configuration once we reexec in waitmode]
Signed-off-by: William Lallemand <wlallemand@haproxy.org>