Avoid boolean tautologies of the form 'not x == y'

Lua operator 'not' has higher precedence than '==', so the statement

    not x == "something"

is equivalent to:

    (not x) == "something"

which will always be false, since the value of 'not x' will be either
'true' or 'false' and the string "something" is not the boolean 'true'
or 'false'. This is usually resolved by using the '~=' operator.

scripts/broadcast-ping.nse

@@ -222,7 +222,7 @@ action = function()
     -- single interface defined
     local interface = interface_opt or interface_arg
     local if_table = nmap.get_interface_info(interface)
-    if not if_table or not if_table.address or not if_table.link=="ethernet" then
+    if not (if_table and if_table.address and if_table.link=="ethernet") then
       stdnse.debug1("Interface not supported or not properly configured.")
       return false
     end

scripts/dns-fuzz.nse

@@ -193,7 +193,7 @@ function dropByte (dnsPacket)
   -- Iterate over every byte in the packet
   dnsPacket:gsub(".", function(c)
       i=i+1
-      if not i==byteToDrop then
+      if i ~= byteToDrop then
         newPacket[#newPacket+1] = c
       end
     end)
@@ -328,7 +328,7 @@ action = function(host, port)
     query =  makePacket ()
     -- induce random jitter
     retStr = corruptAndSend (host, port, query)
-    if not retStr==nil then
+    if retStr then
       return retStr
     end
   end

scripts/lltd-discovery.nse

@@ -268,7 +268,7 @@ action = function()
     -- single interface defined
     local interface = interface_opt or interface_arg
     local if_table = nmap.get_interface_info(interface)
-    if not if_table or not if_table.address or not if_table.link=="ethernet" then
+    if not (if_table and if_table.address and if_table.link=="ethernet") then
       stdnse.debug1("Interface not supported or not properly configured.")
       return false
     end