Implement core IRC capability handling and "CAP" command

This patch implements the core functions to support "IRC Capabilities"
and the IRC "CAP" command as used by other servers and specified here:
<http://www.leeh.co.uk/draft-mitchell-irc-capabilities-02.html>.

It enables ngIRCd to support the defined handshake, but it doesn't
implement any capabilities, so "CAP LS" and "CAP LIST" always return
the empty set and "CAP REQ ..." always fails with "CAP NAK".

doc/Capabilities.txt

@@ -0,0 +1,23 @@
+
+                     ngIRCd - Next Generation IRC Server
+                           http://ngircd.barton.de/
+
+               (c)2001-2012 Alexander Barton and Contributors.
+               ngIRCd is free software and published under the
+                   terms of the GNU General Public License.
+
+                            -- Capabilities.txt --
+
+
+This document lists and describes the "IRC capabilities" that ngIRCd supports
+and can be requested by a IRC/IRCv3 client that supports the "CAP" command.
+
+ngIRCd implements the "IRC Client Capabilities Extension" as described here:
+<http://www.leeh.co.uk/draft-mitchell-irc-capabilities-02.html>
+
+
+I. Supported Capabilities
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+None. At the moment, ngIRCd supports the "CAP" command and its sub-commands
+but offers no capabilities that could be requested by a client.

doc/Makefile.am

@@ -18,6 +18,7 @@ SUFFIXES = .tmpl
 
 static_docs = \
 	Bopm.txt \
+	Capabilities.txt \
 	FAQ.txt \
 	GIT.txt \
 	HowToRelease.txt \