Attr: Duplicated &is_used is allowed

When using the same function for eval conditions in signatures, we
previously attempted to add &is_used multiple times to the function
triggering an ambiguous attribute error.

Turns out there's already a list of attributes that are accepted
to be used multiple times, so just add ATTR_IS_USED there.

Fixes #2628

src/Attr.cc

@@ -224,7 +224,7 @@ void Attributes::AddAttr(AttrPtr attr, bool is_redef)
 
 		return new_tag == ATTR_LOG || new_tag == ATTR_OPTIONAL || new_tag == ATTR_REDEF ||
 		       new_tag == ATTR_BROKER_STORE_ALLOW_COMPLEX || new_tag == ATTR_RAW_OUTPUT ||
-		       new_tag == ATTR_ERROR_HANDLER;
+		       new_tag == ATTR_ERROR_HANDLER || new_tag == ATTR_IS_USED;
 	};
 
 	// A `redef` is allowed to overwrite an existing attribute instead of

testing/btest/Baseline/signatures.signature-cond-used-twice/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/signatures.signature-cond-used-twice/.stdout

@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+signature_cond, GET /download/CHANGES.bro-aux.tx
+signature_match, GET, GET /download/CHANGES.bro-aux.tx
+signature_cond, HTTP/1.1 200 OK\x0d\x0aDate: Thu, 07 M
+signature_match, STATUS, HTTP/1.1 200 OK\x0d\x0aDate: Thu, 07 M

testing/btest/signatures/signature-cond-used-twice.zeek

@@ -0,0 +1,36 @@
+# @TEST-DOC: The function signature_cond is used in two eval's in test.sig should not fail...
+# @TEST-EXEC: unset ZEEK_ALLOW_INIT_ERRORS; zeek -b %INPUT -r $TRACES/http/get.trace
+# @TEST-EXEC: btest-diff .stderr
+# @TEST-EXEC: btest-diff .stdout
+module SignatureEvalTest;
+
+@load-sigs ./test.sig
+
+event signature_match(state: signature_state, msg: string, data: string)
+        {
+        print "signature_match", msg, data[:32];
+        }
+
+function signature_cond(state: signature_state, data: string): bool
+        {
+        print "signature_cond", data[:32];
+        return T;
+        }
+
+
+@TEST-START-FILE test.sig
+signature my-first-sig {
+        ip-proto == tcp
+        dst-port == 80
+        payload /GET/
+        event "GET"
+        eval SignatureEvalTest::signature_cond
+}
+
+signature my-second-sig {
+        ip-proto == tcp
+        payload /HTTP\/1\.1 [0-9]+/
+        event "STATUS"
+        eval SignatureEvalTest::signature_cond
+}
+@TEST-END-FILE