Call curl_global_init() in _init of plugins using curl

Need to call curl_global_init() or curl_easy_init() during init
for plugins when we're still running single threaded.  This
updates the remaining ones
apache plugin: Call curl_global_init() from the init function.

This is a shot in the dark in trying to address #513. By calling this
from an init() callback, I hope to be initializing the curl and gcrypt
libraries before collectd becomes multi-threaded, avoiding the problems
described in the issue.