Add a new expert info type for decryption problems.

This is what should be used if, for some reason, decryption can't be
performed.  (And if there's a known reason why decryption can't be
performed, it should be used, so the user knows why their
802.11/SSL/whatever traffic isn't decrypted, and either doesn't have to
ask why or, at least, can give more details when they do ask why.)

(Yes, I plan to use this for the 802.11 decryption code.  Work in
progress.)

Change-Id: I812e61c2a4613d2e85f9ced1f5ed6ae91ac5f7ae
Reviewed-on: https://code.wireshark.org/review/10421
Reviewed-by: Guy Harris <guy@alum.mit.edu>

epan/expert.c

@@ -80,6 +80,7 @@ const value_string expert_group_vals[] = {
 	{ PI_PROTOCOL,          "Protocol" },
 	{ PI_SECURITY,          "Security" },
 	{ PI_COMMENTS_GROUP,    "Comment" },
+	{ PI_DECRYPTION,        "Decryption" },
 	{ 0, NULL }
 };
 

epan/proto.h

@@ -742,6 +742,8 @@ typedef proto_node proto_item;
 #define PI_SECURITY             0x0a000000
 /** The protocol field indicates a packet comment */
 #define PI_COMMENTS_GROUP       0x0b000000
+/** The protocol field indicates a decryption problem */
+#define PI_DECRYPTION           0x0c000000
 
 /* add more, see https://wiki.wireshark.org/Development/ExpertInfo */