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

form.c

@@ -9,6 +9,7 @@
 #include "myctype.h"
 #include "local.h"
 #include "regex.h"
+#include "util.h"
 
 extern Str *textarea_str;
 extern int max_textarea;
@@ -603,9 +604,8 @@ input_textarea(FormItemList *fi)
 	form_fputs_decode(fi->value, f);
     fclose(f);
 
-    fmTerm();
-    system(myEditor(Editor, tmpf, 1)->ptr);
-    fmInit();
+    if (exec_cmd(myEditor(Editor, tmpf, 1)->ptr))
+	    goto input_end;
 
     if (fi->readonly)
 	goto input_end;