BUG/MINOR: trace: stop rewriting argv with -dt

BUG/MINOR: trace: stop rewriting argv with -dt

When using trace with -dt, the trace_parse_cmd() function is doing a
strtok which write \0 into the argv string.

When using the mworker mode, and reloading, argv was modified and the
trace won't work anymore because the first : is replaced by a '\0'.

This patch fixes the issue by allocating a temporary string so we don't
modify the source string directly. It also replace strtok by its
reentrant version strtok_r.

Must be backported as far as 2.9.

(cherry picked from commit 596db3ef86844617565a0b4b4ce8358fe6537d87)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>