luci-proto-ppp: align keepalive defaults with current OpenWrt

Disabling LCP echos by *not* setting "option keepalive" was broken in
OpenWrt since https://dev.archive.openwrt.org/ticket/2373.html so setting
"0" in LuCI had the effect of reverting back to "5, 1" while the help
suggested otherwise.

Support for "keepalive 0" was fixed with https://git.openwrt.org/555c59230
so align LuCI now to emit "option keepalive 0" instead of removing the
option when entering "0" in the gui.

Fixes #2112
Signed-off-by: Jo-Philipp Wich <jo@mein.io>