Remove instances in libssl of the constant 28 (for size of IPv4 header + UDP)

and instead use the value provided by the underlying BIO. Also provide some
new DTLS_CTRLs so that the library user can set the mtu without needing to
know this constant. These new DTLS_CTRLs provide the capability to set the
link level mtu to be used (i.e. including this IP/UDP overhead). The previous
DTLS_CTRLs required the library user to subtract this overhead first.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(cherry picked from commit 59669b6abf)

Conflicts:
	ssl/d1_both.c
	ssl/ssl_lib.c

ssl/ssl.h

@@ -693,6 +693,10 @@ struct ssl_session_st
         SSL_ctrl((ssl),SSL_CTRL_MODE,0,NULL)
 #define SSL_set_mtu(ssl, mtu) \
         SSL_ctrl((ssl),SSL_CTRL_SET_MTU,(mtu),NULL)
+#define DTLS_set_link_mtu(ssl, mtu) \
+        SSL_ctrl((ssl),DTLS_CTRL_SET_LINK_MTU,(mtu),NULL)
+#define DTLS_get_link_min_mtu(ssl) \
+        SSL_ctrl((ssl),DTLS_CTRL_GET_LINK_MIN_MTU,0,NULL)
 
 #define SSL_get_secure_renegotiation_support(ssl) \
 	SSL_ctrl((ssl), SSL_CTRL_GET_RI_SUPPORT, 0, NULL)
@@ -1632,6 +1636,8 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 #define SSL_CTRL_CLEAR_EXTRA_CHAIN_CERTS	83
 
 #define SSL_CTRL_CHECK_PROTO_VERSION		119
+#define DTLS_CTRL_SET_LINK_MTU			120
+#define DTLS_CTRL_GET_LINK_MIN_MTU		121
 
 #define DTLSv1_get_timeout(ssl, arg) \
 	SSL_ctrl(ssl,DTLS_CTRL_GET_TIMEOUT,0, (void *)arg)