Add support for systemf(), formatted system() replacement

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/systemf.c

@@ -0,0 +1,31 @@
+#include <err.h>
+#include "check.h"
+
+int main(void)
+{
+	struct { char *cmd; int rc; } list[] = {
+		{ "false",          1 },
+		{ "true",           0 },
+		{ "kill -9 $$",     1 },
+		{ "kill -QUIT $$", -1 },
+		{ "kill -INT $$",  -1 }
+	};
+
+	for (size_t i = 0; i < NELEMS(list); i++) {
+		int rc;
+
+		rc = systemf("%s", list[i].cmd);
+		if (rc != list[i].rc)
+			err(rc, "Failed command %s", list[i].cmd);
+	}
+
+	return 0;
+}
+
+/**
+ * Local Variables:
+ *  indent-tabs-mode: t
+ *  c-file-style: "linux"
+ * End:
+ */
+