improve PONG sending

improve PONG sending

this gets rid of the double-send(2) (which in theory allows us to enable
Nagle's TCP_NODELAY). we need to restore the line terminator that strtok
removed. while the spec says we should send \r\n, it would probably
suffice to non-compliantly just do line[n] = '\n' (and rely on postel's
law to save us). but strtok only overwrites the first delimeter;
transforming "...\n..." into "...\0..." and "...\r\n..." into
"...\0\n...". so by looking ahead one byte we can determine which
terminator(s) were sent by the server and send back the same one(s). of
course, this doesn't hold for \n\r, just \r or \n\n (neither of which we
expect), but it would fall back to a sensible form regardless.