ppp: remove hardcoded lcp-echo-failure, lcp-echo-interval values

ppp: remove hardcoded lcp-echo-failure, lcp-echo-interval values

OpenWrt used to ship hardcoded defaults for lcp-echo-failure and
lcp-echo-interval in the non-uci /etc/ppp/options file.

These values break uci support for *disabling* LCP echos through
the use of "option keepalive 0" as either omitting the keepalive
option or setting it to 0 will result in no lcp-echo-* flags
getting passed to the pppd cmdline, causing the pppd process to
revert to the defaults in /etc/ppp/options.

Address this issue by letting the uci "keepalive" option default
to the former hardcoded values "5, 1" and by removing the fixed
lcp-echo-failure and lcp-echo-interval settings from the
/etc/ppp/options files.

Ref: https://github.com/openwrt/luci/issues/2112
Ref: https://dev.archive.openwrt.org/ticket/2373.html
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=854
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=1259
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from 555c592304023a0d24216a6d8ed9d525602ae218)