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.

scripts/imap-capabilities.nse

@@ -0,0 +1,42 @@
+description = [[
+Retrieves IMAP email server capabilities.
+
+IMAP4rev1 capabilities are defined in RFC 3501. The CAPABILITY command
+allows a client to ask a server what commands it supports and possibly
+any site-specific policy.
+]]
+
+---
+-- @output
+-- 143/tcp open  imap
+-- |_ imap-capabilities: LOGINDISABLED IDLE IMAP4 LITERAL+ STARTTLS NAMESPACE IMAP4rev1
+
+
+author = "Brandon Enright <bmenrigh@ucsd.edu>"
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+
+categories = {"default"}
+
+require 'imap'
+require 'shortport'
+require 'stdnse'
+
+portrule = shortport.port_or_service({143}, "imap")
+
+action = function(host, port)
+  local capa, err = imap.capabilities(host, port)
+  if type(capa) == "table" then
+     -- Convert the capabilities table into an array of strings.
+     local capstrings = {}
+     local cap, args
+     for cap, args in pairs(capa) do
+	table.insert(capstrings, cap)
+     end
+     return stdnse.strjoin(" ", capstrings)
+  elseif type(err) == "string" then
+     stdnse.print_debug(1, "%s: '%s' for %s", filename, err, host.ip)
+     return
+  else
+     return "server doesn't support CAPABILITIES"
+  end
+end