KDB limit webapp to select existing tables

blah.q

/ patch HTTP GET/POST handler to not run arbitary Q code from the url
/ this is the minimal handler to keep the built-in UI functional. It evaluates queries
/ that only match whole tables and global variables from the root namespace
.h.val:{-2 .Q.s1 x;$[x in (string each tables `.),string each system "v";:value x;'protect]}