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/pid.c

@@ -22,6 +22,8 @@
  * THE SOFTWARE.
  */
 
+#include "config.h"		/* Generated by configure script */
+
 #include <errno.h>
 #include <paths.h>
 #include <stdio.h>