Adding the imap-capabilities script and supporting imap library.

The imap-capabilities script is mostly feature-complete but I could
see adding some analysis code to warn users of non-SSL'd IMAP servers
that offer STARTTLS without NOLOGIN.

The imap "library" is really a joke.  It does the minimum required to
support getting capabilities and nothing more.  IMAP requires each
command to use a unique identifier like 000, 001, 002, etc.  Right now
the identifier is hardcoded to a001.  To make a real imap library that
supports logging in, and other IMAP features a state variable will
have to be maintained to change the command uid.  It would be nice to
see the library get updated so that IMAP brute-forcing could be
supported.

nselib/imap.lua

@@ -0,0 +1,45 @@
+--- IMAP functions.
+-- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html
+
+module(... or "imap", package.seeall)
+
+require 'stdnse'
+
+
+---
+-- Asks an IMAP server for capabilities.
+--
+-- See RFC 3501.
+-- @param host Host to be queried.
+-- @param port Port to connect to.
+-- @return Table containing capabilities or nil on error.
+-- @return nil or String error message.
+function capabilities(host, port)
+   local socket = nmap.new_socket()
+   local capas = {}
+   socket:set_timeout(10000)
+   local proto = (port.version and port.version.service_tunnel == "ssl" and "ssl") or "tcp"
+   if not socket:connect(host.ip, port.number, proto) then return nil, "Could Not Connect" end
+
+   status, line = socket:receive_lines(1)
+   if not string.match(line, "^[%*] OK") then return nil, "No Response" end
+   
+   socket:send("a001 CAPABILITY\r\n")
+   status, line = socket:receive_buf("\r\n", false)
+   if not status then 
+      capas.CAPABILITY = false
+   else 
+      while status do
+         if string.match(line, "^%*%s+CAPABILITY") then
+	    line = string.gsub(line, "^%*%s+CAPABILITY", "")
+	    for capability in string.gmatch(line, "[%w%+=-]+") do
+	       capas[capability] = true
+            end
+            break
+         end
+         status, line = socket:receive_buf("\r\n", false)
+      end
+   end
+   socket:close()
+   return capas
+end