ppp: support adaptive LCP echos

Port Debians adaptive LCP echo patch to pppd, make it configurable with UCI
and enable it by default.

When adaptive LCP echo is enabled, LCP echo requests are only sent if the
link is idle, this avoids 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.

Also bump the copyright year in the Makefile, remove a redundant maintainer
entry and fix the shell processing of the keepalive option when the two-
value syntax is used.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43143 3c298f89-4303-0410-b956-a3cf2f4a3e73