keepalive: add first basic man pages

Someone on IRC pointed out that we don't have these documented so I
wrote up a first set based on the information in the wiki:
http://trac.libssh2.org/wiki/KeepAlive

docs/libssh2_keepalive_send.3

@@ -0,0 +1,20 @@
+.\" $Id: keepalive_send.3,v 1.4 2007/06/13 16:41:33 jehousley Exp $
+.\"
+.TH libssh2_keepalive_send 3 "13 Apr 2011" "libssh2 1.2.5" "libssh2 manual"
+.SH NAME
+libssh2_keepalive_send - short function description
+.SH SYNOPSIS
+#include <libssh2.h>
+.nf
+
+int libssh2_keepalive_send(LIBSSH2_SESSION *session,
+                           int *seconds_to_next);
+.SH DESCRIPTION
+Send a keepalive message if needed.  \fBseconds_to_next\fP indicates how many
+seconds you can sleep after this call before you need to call it again.
+.SH RETURN VALUE
+Returns 0 on success, or LIBSSH2_ERROR_SOCKET_SEND on I/O errors.
+.SH AVAILABILITY
+Added in libssh2 1.2.5
+.SH SEE ALSO
+.BR libssh2_keepalive_config(3)