BUG/MINOR: startup: haproxy -s cause 100% cpu

BUG/MINOR: startup: haproxy -s cause 100% cpu

It was reported in bug #837 that haproxy -s causes a 100% CPU.

However this option does not exist and haproxy must exit with the
usage message.

The parser was not handling the case where -s is not followed by 't' or
'f' which are the only two valid cases.

This bug was introduced by df6c5a ("BUG/MEDIUM: mworker: fix the copy of
options in copy_argv()") which was backported as far as 1.8.

This fix must be backported as far as 1.8.