Allow for setting of custom timeout for readtcp()

The readtcp() function currently uses the default network timeout value set by default_timeout. It seems it is not possible to set the timeout for readtcp() without having it affect all network connections in the policy. This limits the usefulness of readtcp() for testing whether a server or service is available. Ideally it should be possible to set a custom timeout for readctp() so that, for example, a low timeout value can be set for testing a connection in a way that won't dramatically slow down execution of policies.