Drop config.h include from helpers.h, exported header file

Drop the config.h include from helpers.h after a report from a colleague
trying to build an external plugin from the latest GIT sources.

Instead, make sure config.h is included, and properly commented, in all
.c files that have configure #ifdefs and other deps.  Also, move more
ot the includes from helpers.h to their respective .c file instead to
reduce the amount of headers an external plugin pulls in.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/plugin.c

@@ -21,6 +21,8 @@
  * THE SOFTWARE.
  */
 
+#include "config.h"		/* Generated by configure script */
+
 #include <errno.h>
 #include <dlfcn.h>		/* dlopen() et al */
 #include <dirent.h>		/* readdir() et al */
@@ -34,7 +36,6 @@
 # include <lite/queue.h>	/* BSD sys/queue.h API */
 #endif
 
-#include "config.h"
 #include "cond.h"
 #include "finit.h"
 #include "helpers.h"