Prefer explicit construction to coercion in record initialization

While we support initializing records via coercion from an expression
list, e.g.,

    local x: X = [$x1=1, $x2=2];

this can sometimes obscure the code to readers, e.g., when assigning to
value declared and typed elsewhere. The language runtime has a similar
overhead since instead of just constructing a known type it needs to
check at runtime that the coercion from the expression list is valid;
this can be slower than just writing the readible code in the first
place, see #4559.

With this patch we use explicit construction, e.g.,

    local x = X($x1=1, $x2=2);

scripts/base/protocols/ftp/utils-commands.zeek

@@ -80,7 +80,7 @@ export {
 
 function add_pending_cmd(pc: PendingCmds, seq: count, cmd: string, arg: string): CmdArg
 	{
-	local ca = [$cmd = cmd, $arg = arg, $seq=seq, $ts=network_time()];
+	local ca = CmdArg($cmd = cmd, $arg = arg, $seq=seq, $ts=network_time());
 	pc[ca$seq] = ca;
 
 	return ca;