pppd: Add lcp-echo-adaptive option

This adds an option that has been added by Debian and other distros
for a while now.

When adaptive LCP echo is enabled, LCP echo requests are only sent if the
link is idle, avoiding the common situation where a congested PPP link
(e.g. during torrenting) is falsely detected as disconnected because the
LCP replies are not received in time.

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