get table prefix more accurately

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@767034 b8457f37-d9ea-0310-8a92-e5e31aec5664
update phpseclib

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@767035 b8457f37-d9ea-0310-8a92-e5e31aec5664