uipc_socket.c: Modify MSG_TLSAPPDATA to only do Alert Records

Without this patch, the MSG_TLSAPPDATA flag would cause
soreceive_generic() to return ENXIO for any non-application
data record in a TLS receive stream.

This works ok for TLS1.2, since Alert records appear to be
the only non-application data records received.
However, for TLS1.3, there can be post-handshake handshake
records, such as NewSessionKey sent to the client from the
server. These handshake records cannot be handled by the
upcall which does an SSL_read() with length == 0.

It appears that the client can simply throw away these
NewSessionKey records, but to do so, it needs to receive
them within the kernel.

This patch modifies the semantics of MSG_TLSAPPDATA slightly,
so that it only applies to Alert records and not Handshake
records. It is needed to allow the krpc to work with KTLS1.3.

Reviewed by:	hselasky
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D35170

rtemsbsd/include/machine/_kernel_socket.h

@@ -46,7 +46,7 @@
 #define	MSG_SOCALLBCK    0x00010000	/* for use by socket callbacks - soreceive (TCP) */
 
 #define	MSG_MORETOCOME	 0x00100000	/* additional data pending */
-#define	MSG_TLSAPPDATA	 0x00200000	/* only soreceive() app. data (TLS) */
+#define	MSG_TLSAPPDATA	 0x00200000	/* do not soreceive() alert rec. (TLS) */
 
 #define	CMSG_ALIGN(n)	_ALIGN(n)