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/stty.c

@@ -21,10 +21,12 @@
  * THE SOFTWARE.
  */
 
+#include "config.h"		/* Generated by configure script */
+
 #include <errno.h>
 #include <stdlib.h>
-#include <unistd.h>	      /* _POSIX_VDISABLE */
-#include <sys/ttydefaults.h>  /* Not included by default in musl libc */
+#include <unistd.h>		/* _POSIX_VDISABLE */
+#include <sys/ttydefaults.h>	/* Not included by default in musl libc */
 #include <termios.h>
 
 #include "helpers.h"