Minor fixes to new shell feature

The check for the system() exit status is slightly problematic, because
bash returns the status code of the last command it executed. I've set
it to only check for status code 127 now (command not found) in order to
at least provide a message when the $SHELL command can't be found. This
error can still be triggered when executing a nonexistant command within
the shell and then exiting.

src/global.h

@@ -103,5 +103,6 @@ int input_handle(int);
 #include "help.h"
 #include "path.h"
 #include "util.h"
+#include "shell.h"
 
 #endif

src/help.c

@@ -32,7 +32,7 @@
 int page, start;
 
 
-#define KEYS 16
+#define KEYS 17
 char *keys[KEYS*2] = {
 /*|----key----|  |----------------description----------------|*/
         "up, k", "Move cursor up",

src/shell.c

@@ -34,7 +34,7 @@
 #include <unistd.h>
 
 void shell_draw() {
-  char *full_path, *shell;
+  char *full_path;
   int res;
 
   /* suspend ncurses mode */
@@ -60,7 +60,7 @@ void shell_draw() {
     /* resume ncurses mode */
     reset_prog_mode();
 
-    if (res == -1 || WEXITSTATUS(res) != 0) {
+    if (res == -1 || !WIFEXITED(res) || WEXITSTATUS(res) == 127) {
       clear();
       printw("ERROR: Can't execute shell interpreter: %s\n"
              "\n"