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 it's perror() implementation translates the errno to
a message, it is printed like:

	exec /runc.armel: exec format error

Note that when not using nolibc, the error is printed in the same way as
before.

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

libcontainer/dmz/_dmz.c

@@ -1,5 +1,7 @@
 #ifdef RUNC_USE_STDLIB
 #  include <unistd.h>
+#  include <stdio.h>
+#  include <string.h>
 #else
 #  include "xstat.h"
 #  include "nolibc/nolibc.h"
@@ -11,5 +13,17 @@ int main(int argc, char **argv)
 {
 	if (argc < 1)
 		return 127;
-	return execve(argv[0], argv, environ);
+	int r = execve(argv[0], argv, environ);
+	if (r) {
+		char *prefix = "exec ";
+		int err_len = strlen(prefix) + strlen(argv[0]) + 1;
+		char err[err_len];
+
+		strcpy(err, prefix);
+		strcpy(err + strlen(prefix), argv[0]);
+		err[err_len - 1] = '\0';
+
+		perror(err);
+	}
+	return r;
 }