implement logging the LCP RTT

This change adds the lcp-rtt-file configuration option, which instructs
pppd to add a timestamp to the data section of each LCP echo request
frame and then log their round-trip time and any detected packet loss
to a circular buffer in that file.

Other programs then can asynchronously read the file and report
statistics about the line.

Signed-off-by: Marco d'Itri <md@linux.it>