Refactor some Lua string concatenations.

Using this regular expression, '\(\w*\)\s*=\s*\1\s*\.\.', found and
replaced many string concatenation-reassignments. These can cause
performance issues, since a new string gets allocated for each
reassignment. In many cases, the replacement is simply a single string,
wrapped across lines with the '\z' escape, which consumes a newline and
whitespace following it. In other cases, a table is used to hold the
substrings until the final string is built with a single table.concat
operation (same technique used in stdnse.strbuf).

Also, some string-building loops of this form:

s = ""
for i = 1, 100, 1 do
  s = s .. "\0"
end

were replaced with this much faster and cleaner version:

s = string.rep("\0", 100)

nselib/url.lua

@@ -290,29 +290,27 @@ end
 -- @return The corresponding path string
 -----------------------------------------------------------------------------
 function build_path(parsed, unsafe)
-  local path = ""
+  local path = {}
+  if parsed.is_absolute then path[#path+1] = "/" end
   local n = #parsed
   if unsafe then
     for i = 1, n-1 do
-      path = path .. parsed[i]
-      path = path .. "/"
+      path[#path+1] = parsed[i] .. "/"
     end
     if n > 0 then
-      path = path .. parsed[n]
-      if parsed.is_directory then path = path .. "/" end
+      path[#path+1] = parsed[n]
+      if parsed.is_directory then path[#path+1] = "/" end
     end
   else
     for i = 1, n-1 do
-      path = path .. protect_segment(parsed[i])
-      path = path .. "/"
+      path[#path+1] = protect_segment(parsed[i]) .. "/"
     end
     if n > 0 then
-      path = path .. protect_segment(parsed[n])
-      if parsed.is_directory then path = path .. "/" end
+      path[#path+1] = protect_segment(parsed[n])
+      if parsed.is_directory then path[#path+1] = "/" end
     end
   end
-  if parsed.is_absolute then path = "/" .. path end
-  return path
+  return table.concat(path)
 end
 
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