Handle failed system calls

Introduce a separate TU for utility functions util.c. Add a function
exec_cmd to simplify execution of system commands with error handling.

While at it, suppress a warning about unused result when executing a
shell command. As we only display the command's output we do not care
about the exit code.

This fixes Debian bug #398989[0]

[0]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=398989

util.c

@@ -0,0 +1,26 @@
+#include "util.h"
+
+#include "display.h"
+#include "terms.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int
+exec_cmd(char *cmd)
+{
+    int rv;
+
+    fmTerm();
+    if ((rv = system(cmd))) {
+	printf("\n[Hit any key]");
+	fflush(stdout);
+	fmInit();
+	getch();
+
+	return rv;
+    }
+    fmInit();
+
+    return 0;
+}