Add @deprecated directive

It emits a warning stating that the script is deprecated.

doc/script-reference/directives.rst

@@ -27,6 +27,16 @@ executed.  Directives are evaluated before script execution begins.
 
         print "File:", @FILENAME;
 
+.. bro:keyword:: @deprecated
+
+    Marks the current script as deprecated. This can be placed anywhere in
+    the script, but a good convention is to put it as the first line.
+    You can also supply additional comments.
+
+    Example::
+
+        @deprecated "Use '@load foo' instead"
+
 .. bro:keyword:: @load
 
     Loads the specified Bro script, specified as the relative pathname

src/scan.l

@@ -315,6 +315,23 @@ when	return TOK_WHEN;
 	return TOK_ATTR_SYNCHRONIZED;
 	}
 
+@deprecated.* {
+	auto num_files = file_stack.length();
+	auto comment = skip_whitespace(yytext + 11);
+
+	if ( num_files > 0 )
+		{
+		auto lf = file_stack[num_files - 1];
+
+		if ( lf->name )
+			reporter->Warning("deprecated script loaded from %s:%d %s",
+			                  lf->name, lf->line, comment);
+		else
+			reporter->Warning("deprecated script loaded from command line arguments %s", comment);
+		}
+	else
+		reporter->Warning("deprecated script loaded %s", comment);
+	}
 
 @DEBUG	return TOK_DEBUG;	// marks input for debugger
 

testing/btest/Baseline/language.at-deprecated/.stderr

@@ -0,0 +1,3 @@
+warning in ./foo.bro, line 1: deprecated script loaded from command line arguments 
+warning in ./bar.bro, line 1: deprecated script loaded from ./foo.bro:2 "Use '@load qux.bro' instead"
+warning in ./baz.bro, line 1: deprecated script loaded from ./foo.bro:3 

testing/btest/language/at-deprecated.bro

@@ -0,0 +1,16 @@
+# @TEST-EXEC: bro -b foo
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+@TEST-START-FILE foo.bro
+@deprecated
+@load bar
+@load baz
+@TEST-END-FILE
+
+@TEST-START-FILE bar.bro
+@deprecated "Use '@load qux.bro' instead"
+@TEST-END-FILE
+
+@TEST-START-FILE baz.bro
+@deprecated
+@TEST-END-FILE