Move agent_named_pipe_name into its own source file.

It's used by server and client, so it seems silly to have it live in
the client code.

windows/utils/agent_named_pipe_name.c

@@ -0,0 +1,17 @@
+/*
+ * Return the full pathname of the named pipe Pageant will listen on.
+ * Used by both the Pageant server code and client code.
+ */
+
+#include "putty.h"
+#include "cryptoapi.h"
+
+char *agent_named_pipe_name(void)
+{
+    char *username = get_username();
+    char *suffix = capi_obfuscate_string("Pageant");
+    char *pipename = dupprintf("\\\\.\\pipe\\pageant.%s.%s", username, suffix);
+    sfree(username);
+    sfree(suffix);
+    return pipename;
+}