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/helpers.h

@@ -25,10 +25,6 @@
 #ifndef FINIT_HELPERS_H_
 #define FINIT_HELPERS_H_
 
-#include "config.h"
-
-#include <mntent.h>
-#include <sched.h>		/* sched_yield() */
 #include <stdarg.h>
 #include <sys/resource.h>
 #include <sys/types.h>