libct/dmz: Print execve() errors

This error code is using functions that are present in nolibc too.

When using nolibc, the error is printed like:

	exec /runc.armel: errno=8

When using libc, as its perror() implementation translates the errno to
a message, it is printed like:

	exec /runc.armel: exec format error

Note that when using libc, the error is printed in the same way as
before.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

libcontainer/dmz/_dmz.c

@@ -1,4 +1,7 @@
 #ifdef RUNC_USE_STDLIB
+#  include <linux/limits.h>
+#  include <stdio.h>
+#  include <string.h>
 #  include <unistd.h>
 #else
 #  include "xstat.h"
@@ -11,5 +14,15 @@ int main(int argc, char **argv)
 {
 	if (argc < 1)
 		return 127;
-	return execve(argv[0], argv, environ);
+	int ret = execve(argv[0], argv, environ);
+	if (ret) {
+		/* NOTE: This error message format MUST match Go's format. */
+		char err_msg[5 + PATH_MAX] = "exec ";	// "exec " + argv[0]
+
+		strncpy(err_msg + strlen(err_msg), argv[0], PATH_MAX);
+		err_msg[sizeof(err_msg) - 1] = '\0';
+
+		perror(err_msg);
+	}
+	return ret;
 }