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>

plugins/bootmisc.c

@@ -21,8 +21,12 @@
  * THE SOFTWARE.
  */
 
+#include "config.h"		/* Generated by configure script */
+
 #include <ftw.h>
+#ifdef HAVE_MNTENT_H
 #include <mntent.h>
+#endif
 #include <string.h>
 #ifdef _LIBITE_LITE
 # include <libite/lite.h>