BUG/MAJOR: spoe: Fix initialization of thread-dependent fields

BUG/MAJOR: spoe: Fix initialization of thread-dependent fields

A bug was introduced in the commit b0769b ("BUG/MEDIUM: spoe: initialization
depending on nbthread must be done last"). The code depending on global.nbthread
was moved from cfg_parse_spoe_agent() to spoe_check() but the pointer on the
agent configuration was not updated to use the filter's one. The variable
curagent is a global variable only valid during the configuration parsing. In
spoe_check(), conf->agent must be used instead.

This patch must be backported to 1.9 and 1.8.