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.h

@@ -0,0 +1,6 @@
+#ifndef W3M_UTIL_H
+#define W3M_UTIL_H
+
+int exec_cmd(char *cmd);
+
+#endif