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
indent wh_init() to be consistent with the rest of the file
Configparser: when we alocate an empty list, we also need to reset the counter; else we will trip over this later.
This fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=750440
don't assume pkg-config is in PATH
add missing backticks which broke the build
snmp: free snmp_pdu struct allocated by snmp_pdu_create()

This should fix the leak reported in issue #610.
curl_xml plugin: Fixed tautological pointer comparison error.

Fixes: http://bugs.debian.org/758481
Signed-off-by: Florian Forster <octo@collectd.org>