BUG/MINOR: flt-trace: Support only one name option

BUG/MINOR: flt-trace: Support only one name option

When a trace filter is defined, only one 'name' option is expected. But it
was not tested. Thus it was possible to set several names leading to a
memory leak.

It is now tested, and it is not allowed to redefine the trace filter name.

It was reported by Coverity in #2841: CID 1587768.

This patch could be backported to all stable versions.

(cherry picked from commit b20e2c96cfea06776db3200614f6d94441f15c2d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit e3e565a556ce960ac47f4495687a0bbda38ae160)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>