Fix usage of nmap.fetchfile in several scripts

Discussion thread: http://seclists.org/nmap-dev/2013/q2/121

Existing behavior preserved and preferred, but absolute and local paths
should also work now. Notably, smb-psexec's locate_file function claimed
to check current directory but did not.

scripts/http-sql-injection.nse

@@ -208,7 +208,7 @@ end
 
 -- load error strings to the errorstrings table
 local function get_error_strings(path)
-  local f = nmap.fetchfile(path)
+  local f = nmap.fetchfile(path) or path
   if f then
     for e in io.lines(f) do
       if not string.match(e, "^#") then